[UNRELEASED]
Issue file delete only once per unique style when nullifying attachment or destroying an object. Avoids triggering a rate limit error on Google Cloud Storage.

7.0.0 (2021-05-28)
* Replace `mimemagic` gem with `marcel` due to licensing issues. See https://github.com/kreeti/kt-paperclip/pull/54 for details and limitations

6.4.1 (2021-01-30)
* Improvement: Use URI.open in ruby > 2.5 (PR #45)

6.4.0 (2020-12-14)
* Improvement: Fix Ruby 2.7 method & syntax deprecation warnings (#38)

6.3.0 (2020-08-10)
* Feature: Add validation_errors_to option - customize copying of errors (#26)
* Stability: Dropped support for ruby 2.1

6.2.2 (2020-02-18)
* Bugfix: Add support for Aliases in S3 credentials YAML (accidentally removed during 6.2.0 upgrade).

6.2.1 (2020-01-30)

* Bugfix: Auto-loading of the gem code (necessitated due to change in gem name).

6.2.0 (2020-01-23):

* Bugfix: Don't do post-processing on invalid attachments (#16).
* Improvement: Use AWS::S3 built in download_file method (#9).
* Stability: Rubocop recommended syntax updates (#10, #11, #13, #14).
* Stability: Testing - replace mocha and bourne with rspec mocks (#6).
* Stability: Fix test suite to run with Rails 5 and Rails 6 (#19, #20, #7).

6.1.0 (2018-07-27):

* BUGFIX: Don't double-encode URLs (Roderick Monje).
* BUGFIX: Only use the content_type when it exists (Jean-Philippe Doyle).
* STABILITY: Better handling of the content-disposition header. Now supports
  file name that is either enclosed or not in double quotes and is case
  insensitive as per RC6266 grammar (Hasan Kumar, Yves Riel).
* STABILITY: Change database column type of attachment file size from unsigned 4-byte
  `integer` to unsigned 8-byte `bigint`. The former type limits attachment size
  to just over 2GB, which can easily be exceeded by a large video file (Laurent
  Arnoud, Alen Zamanyan).
* STABILITY: Better error message when thumbnail processing errors (Hayden Ball).
* STABILITY: Fix file linking issues around Windows (Akihiko Odaki).
* STABILITY: Files without an extension will now be checked for spoofing attempts
  (George Walters II).
* STABILITY: Manually close Tempfiles when we are done with them (Erkki Eilonen).

6.0.0 (2018-03-09):

* Improvement: Depend only on `aws-sdk-s3` instead of `aws-sdk` (https://github.com/thoughtbot/paperclip/pull/2481)

5.3.0 (2018-03-09):

* Improvement: Use `FactoryBot` instead of `FactoryGirl` (https://github.com/thoughtbot/paperclip/pull/2501)
* Improvement: README updates (https://github.com/thoughtbot/paperclip/pull/2411, https://github.com/thoughtbot/paperclip/pull/2433, https://github.com/thoughtbot/paperclip/pull/2374, https://github.com/thoughtbot/paperclip/pull/2417, https://github.com/thoughtbot/paperclip/pull/2536)
* Improvement: Remove Ruby 2.4 deprecation warning (https://github.com/thoughtbot/paperclip/pull/2401)
* Improvement: Rails 5 migration compatibility (https://github.com/thoughtbot/paperclip/pull/2470)
* Improvement: Documentation around post processing (https://github.com/thoughtbot/paperclip/pull/2381)
* Improvement: S3 hostname example documentation (https://github.com/thoughtbot/paperclip/pull/2379)
* Bugfix: Allow paperclip to load in IRB (https://github.com/thoughtbot/paperclip/pull/2369)
* Bugfix: MIME type detection (https://github.com/thoughtbot/paperclip/issues/2527)
* Bugfix: Bad tempfile state after symlink failure (https://github.com/thoughtbot/paperclip/pull/2540)
* Bugfix: Rewind file after Fog bucket creation (https://github.com/thoughtbot/paperclip/pull/2572)
* Improvement: Use `Terrapin` instead of `Cocaine` (https://github.com/thoughtbot/paperclip/pull/2553)

5.2.1 (2018-01-25):

* Bugfix: Fix copying files on Windows. (#2532)

5.2.0 (2018-01-23):

* Security: Remove the automatic loading of URI adapters. Some of these
  adapters can be specially crafted to expose your network topology. (#2435)
* Bugfix: The rake task no longer rescues `Exception`. (#2476)
* Bugfix: Handle malformed `Content-Disposition` headers (#2283)
* Bugfix: The `:only_process` option works when passed a lambda again. (#2289)
* Improvement: Added `:use_accelerate_endpoint` option when using S3 to enable
  [Amazon S3 Transfer Acceleration](http://docs.aws.amazon.com/AmazonS3/latest/dev/transfer-acceleration.html)
  (#2291)
* Improvement: Make the fingerprint digest configurable per attachment. The
  default remains MD5. Making this configurable means it can change in a future
  version because it is not considered secure anymore against intentional file
  corruption. For more info, see https://en.wikipedia.org/wiki/MD5#Security

  You can change the digest used for an attachment by adding the
  `:adapter_options` parameter to the `has_attached_file` options like this:
  `has_attached_file :avatar, adapter_options: { hash_digest: Digest::SHA256 }`

  Use the rake task to regenerate fingerprints with the new digest for a given
  class. Note that this does **not** check the file integrity using the old
  fingerprint. Run the following command to regenerate fingerprints for all
  User attachments:
  `CLASS=User rake paperclip:refresh:fingerprints`
  You can optionally limit the attachment that will be processed, e.g:
  `CLASS=User ATTACHMENT=avatar rake paperclip:refresh:fingerprints` (#2229)
* Improvement: The new `frame_index` option on the thumbnail processor allows
  you to select a specific frame from an animated upload to use as a thumbnail.
  Initial support is for mkv, avi, MP4, mov, MPEG, and GIF. (#2155)
* Improvement: Instead of copying files, use hard links. This is an
  optimization. (#2120)
* Improvement: S3 storage option `:s3_prefixes_in_alias`. (#2287)
* Improvement: Fog option `:fog_public` can be a lambda. (#2302)
* Improvement: One fewer warning on JRuby. (#2352)
* Ruby 2.4.0 compatibility (doesn't use Fixnum anymore)

5.1.0 (2016-08-19):

* Add default `content_type_detector` to `UploadedFileAdapter` (#2270)
* Default S3 protocol to empty string (#2038)
* Don't write original file if it wasn't reprocessed (#1993)
* Disallow trailing newlines in regular expressions (#2266)
* Support for readbyte in Paperclip attachments (#2034)
* (port from 4.3) Uri io adapter uses the content-disposition filename (#2250)
* General refactors and documentation improvements

5.0.0 (2016-07-01):

* Improvement: Add `read_timeout` configuration for URI Adapter download_content method.
* README adjustments for Ruby beginners (add links, elucidate model in Quick Start)
* Bugfix: Now it's possible to save images from URLs with special characters [#1932]
* Bugfix: Return false when file to copy is not present in cloud storage [#2173]
* Automatically close file while checking mime type [#2016]
* Add `read_timeout` option to `UriAdapter#download_content` method [#2232]
* Fix a nil error in content type validation matcher [#1910]
* Documentation improvements

5.0.0.beta2 (2016-04-01):

* Bugfix: Dynamic fog directory option is now respected
* Bugfix: Fixes cocaine duplicated paths [#2169]
* Removal of dead code (older versions of Rails and AWS SDK)
* README adjustments

5.0.0.beta1 (2016-03-13):

* Bug Fix: megabytes of mime-types info in logs when a spoofed media type is detected.
* Drop support to end-of-life'd ruby 2.0.
* Drop support for end-of-life'd Rails 3.2 and 4.1
* Drop support for AWS v1
* Remove tests for JRuby and Rubinius from Travis CI (they were failing)
* Improvement: Add `fog_options` configuration to send options to fog when
  storing files.
* Extracted repository for locales only:  https://github.com/thoughtbot/paperclip-i18n
* Bugfix: Original file could be unlinked during `post_process_style`, producing failures
* Bugfix for image magick scaling images up
* Memory consumption improvements
* `url` on a unpersisted record returns `default_url` rather than `nil`
* Improvement: aws-sdk v2 support
  https://github.com/thoughtbot/paperclip/pull/1903

  If your Gemfile contains aws-sdk (>= 2.0.0) and aws-sdk-v1, paperclip will use
  aws-sdk v2. With aws-sdk v2, S3 storage requires you to set the s3_region.
  s3_region may be nested in s3_credentials, and (if not nested in
  s3_credentials) it may be a Proc.

4.3

See patch versions in v4.3 NEWS:
https://github.com/thoughtbot/paperclip/blob/v4.3/NEWS

4.3.0 (2015-06-18):

* Improvement: Update aws-sdk and cucumber gem versions.
* Improvement: Add `length` alias for `size` method in AbstractAdapter.
* Improvement: Removed some cruft
* Improvement: deep_merge! Attachment definitions
* Improvement: Switch to mimemagic gem for content-type detection
* Improvement: Allows multiple content types for spoof detector
* Bug Fix: Don't assume we have Rails.env if we have Rails
* Performance: Decrease Memory footprint
* Ruby Versioning: Drop support for 1.9.3 (EOL'ed)
* Rails Versioning: Drop support for 4.0.0 (EOL'ed)

4.2.4 (2015-06-05):

* Rollback backwards incompatible change, allowing paperclip to run on
  Ruby >= 1.9.2.

4.2.3:

* Fix dependency specifications (didn't work with Rails 4.1)
* Fix paperclip tests in CI

4.2.2:

* Security fix: Fix a potential security issue with spoofing

4.2.1:

* Improvement: Added `validate_media_type` options to allow/bypass spoof check
* Improvement: Added incremental backoff when AWS gives us a SlowDown error.
* Improvement: Stream downloads when usign aws-sdk.
* Improvement: Documentation fixes, includes Windows instructions.
* Improvement: Added pt-BR, zh-HK, zh-CN, zh-TW, and ja-JP locales.
* Improvement: Better escaping for characters in URLs
* Improvement: Honor `fog_credentials[:scheme]`
* Improvement: Also look for custom processors in lib/paperclip
* Improvement: id partitioning for string IDs works like integer id
* Improvement: Can pass options to DB adapters in migrations
* Improvement: Update expiring_url creation for later versions of fog
* Improvement: `path` can be a Proc in S3 attachments
* Test Fix: Improves speed and reliability of the specs
* Bug Fix: #original_filename= does not error when passed `nil`

4.2.0:

* Improvement: Converted test suite from test/unit to RSpec
* Improvement: Refactored Paperclip::Attachment#assign
* Improvement: Added Spanish and German locales
* Improvement: Required Validators accept validator subclasses
* Improvement: EXIF orientation checking can be turned off for performance
* Improvement: Documentation updates
* Improvement: Better #human_size method for AttachmentSizeValidators
* Bug Fix: Allow MIME-types with dots in them
* Improvement: Travis CI updates
* Improvement: Validators can take multiple messages
* Improvement: Per-style options for S3 storage
* Improvement: Allow `nil` geometry strings
* Improvement: Use `eager_load!`

4.1.1:

* Improvement: Add default translations for spoof validation
* Bug Fix: Don't check for spoofs if the file hasn't changed
* Bug Fix: Callback chain terminator is different in Rails 4.1, remove warnings
* Improvement: Fixed various Ruby warnings
* Bug Fix: Give bundler a hint, so it doesn't run forever on a fresh bundle
* Improvement: Documentation fixes
* Improvement: Allow travis-ci to finish-fast


4.1.0:

* Improvement: Add :content_type_mappings to correct for missing spoof types
* Improvement: Credit Egor Homakov with discovering the content_type spoof bug
* Improvement: Memoize calls to identify in the thumbnail processor
* Improvement: Make MIME type optional for Data URIs.
* Improvement: Add default format for styles

4.0.0:

* Security: Attachments are checked to make sure they're not pulling a fast one.
* Security: It is now *enforced* that every attachment has a file/mime validation.
* Bug Fix: Removed a call to IOAdapter#close that was causing issues.
* Improvement: Added bullets to the 3.5.3 list of changes. Very important.
* Improvement: Updated the copyright to 2014

3.5.3:

* Improvement: After three long, hard years... we know how to upgrade
* Bug Fix: #expiring_url returns 'missing' urls if nothing is attached
* Improvement: Lots of documentation fixes
* Improvement: Lots of fixes for Ruby warnings
* Improvement: Test the most appropriate Ruby/Rails comobinations on Travis
* Improvement: Delegate more IO methods through IOAdapters
* Improvement: Remove Rails 4 deprecations
* Improvement: Both S3's and Fog's #expiring_url can take a Time or Int
* Bug Fix: Both S3's and Fog's expiring_url respect style when missing the file
* Bug Fix: Timefiles will have a reasonable-length name. They're all MD5 hashes now
* Bug Fix: Don't delete files off S3 when reprocessing due to AWS inconsistencies
* Bug Fix: "swallow_stream" isn't thread dafe. Use :swallow_stderr
* Improvement: Regexps use \A and \Z instead of ^ and $
* Improvement: :s3_credentials can take a lambda as an argument
* Improvement: Search up the class heirarchy for attachments
* Improvement: deep_merge options instead of regular merge
* Bug Fix: Prevent file deletion on transaction rollback
* Test Improvement: Ensure more files are properly closed during tests
* Test Bug Fix: Return the gemfile's syntax to normal

3.5.2:

* Security: Force cocaine to at least 0.5.3 to include a security fix
* Improvement: Fixed some README exmaples
* Feature: Added HTTP URL Proxy Adapter, can assign string URLs as attachments
* Improvement: Put validation errors on the base attribute and the sub-attribute

3.5.1:

* Bug Fix: Returned the class-level `attachment_definitions` method for compatability.
* Improvement: Ensured compatability with Rails 4
* Improvement: Added Rails 4 to the Appraisals
* Bug Fix: #1296, where validations were generating errors
* Improvement: Specify MIT license in the gemspec

3.5.0:

* Feature: Handle Base64-encoded data URIs as uploads
* Feature: Add a FilenameCleaner class to allow custom filename sanitation
* Improvement: Satisfied Mocha deprecation warnings
* Bug Fix: Allow empty string to be submitted and ignored, as some forms do this
* Improvement: Make #expiring_url behavior consistent with #url
* Bug Fix: "Validate" attachments without invoking AR's validations
* Improvement: Various refactorings for a cleaner codebase
* Improvement: Be agnostic, use ActiveModel when appropriate
* Improvement: Add validation errors to the base attachment attribute
* Improvement: Handle errors in rake tasks
* Improvement: Largely refactor has_attached_file into a new class
* Improvement: Added Ruby 2.0.0 as a supported platform and removed 1.8.7
* Improvement: Fixed some incompatabilities in the test suite

3.4.2:

* Improvement: Use https for Gemfile urls
* Improvement: Updated and more correct documentation
* Improvement: Use the -optimize flag on animated GIFs
* Improvement: Remove the Gemfile.lock
* Improvement: Add #expiring_url as an alias for #url until the storage defines it
* Improvement: Remove path clash checking, as it's unnecessary
* Bug Fix: Do not rely on checking version numbers for aws-sdk

3.4.1:

* Improvement: Various documentation fixes and improvements
* Bug Fix: Clearing an attachment with `preserve_files` on should still clear the attachment
* Bug Fix: Instances are #changed? when a new file is assigned
* Bug Fix: Correctly deal with S3 styles when using a lambda
* Improvement: Accept and pass :credential_provider option to AWS-SDK
* Bug Fix: Sanitize original_filename more correctly in IO Adapters
* Improvement: s3_host_name can be a lambda
* Improvement: Cache some interpolations for speed
* Improvement: Update to latest cocaine
* Improvement: Update copyrights, various typos

3.4.0:

* Bug Fix: Allow UploadedFileAdapter to force the use of `file`
* Bug Fix: Close the file handle when dealing with URIs
* Bug Fix: Ensure files are closed for writing when we're done.
* Bug Fix: Fixed 'type' being nil on Windows 7 error.
* Bug Fix: Fixed nil access when no s3 headers are defined
* Bug Fix: Fixes auto_orientation
* Bug Fix: Prevent a missing method error when switching from aws_sdk to fog
* Bug Fix: Properly fail to process invalid attachments
* Bug Fix: Server-side encryption is specified correctly
* Bug Fix: fog_public returned to true by default
* Bug Fix: Check attachment paths for duplicates, not URLs
* Feature: Add Attachment#blank?
* Feature: Add support for blacklisting certain content_types
* Feature: Add support for style-specific s3 headers and meta data
* Feature: Allow only_process to be a lambda
* Feature: Allow setting of escape url as a default option
* Feature: Create :override_file_permissions option for filesystem attachments
* Improvement: Add Attachment#as_json
* Improvement: Evaluate lambdas for fog_file properties
* Improvement: Extract geometry parsing into factories
* Improvement: Fixed various typos
* Improvement: Refactored some tests
* Improvement: Reuse S3 connections

New In 3.3.1:

* Bug Fix: Moved Filesystem's copy_to_local_file to the right place.

3.3.0:

* Improvement: Upgrade cocaine to 0.4

3.2.0:

* Bug Fix: Use the new correct Amazon S3 encryption header.
* Bug Fix: The rake task respects the updated_at column.
* Bug Fix: Strip newline from content type.
* Feature: Fog file visibility can be specified per style.
* Feature: Automatically rotate images.
* Feature: Reduce class-oriented programming of the attachment definitions.

3.1.4:

* Bug Fix: Allow user to be able to set path without `:style` attribute and not raising an error.
  This is a regression introduced in 3.1.3, and that feature will be postponed to another minor
  release instead.
* Feature: Allow for URI Adapter as an optional paperclip io adapter.

3.1.3:

* Bug Fix: Copy empty attachment between instances is now working.
* Bug Fix: Correctly rescue Fog error.
* Bug Fix: Using default path and url options in Fog storage now work as expected.
* Bug Fix: `Attachment#s3_protocol` now returns a protocol without colon suffix.
* Feature: Paperclip will now raise an error if multiple styles are defined but no `:style`
  interpolation exists in `:path`.
* Feature: Add support for `#{attachment}_created_at` field
* Bug Fix: Paperclip now gracefully handles msising file command.
* Bug Fix: `StringIOAdapter` now accepts content type.

3.1.2:

* Bug Fix: #remove_attachment on 3.1.0 and 3.1.1 mistakenly trying to remove the column that has
  the same name as data type (such as :string, :datetime, :interger.) You're advised to update to
  Paperclip 3.1.2 as soon as possible.

3.1.1:

* Bug Fix: Paperclip will only load Paperclip::Schema only when Active Record is available.

3.1.0:

* Feature: Paperclip now support new migration syntax (sexy migration) that reads better:

      class AddAttachmentToUsers < ActiveRecord::Migration
        def self.up
          create_table :users do |t|
            t.attachment :avatar
          end
        end
      end

  Also, schema-definition level syntax has been added:

      add_attachment :users, :avatar
      remove_attachment :users, :avatar

* Feature: Migration now support Rails 3.2+ `change` method.
* API CHANGE: Old `t.has_attached_file` and `drop_attached_file` are now deprecated. You're advised
  to update your migration file before the next MAJOR version.
* Bug Fix: Tempfile now rewinded before generating fingerprint
* API CHANGE: Tempfiles are now unlinked after `after_flush_writes`

  If you need to interact with the generated tempfiles, please define an `after_flush_writes` method
  in your model. You'll be able to access files via `@queue_for_write` instance variable.

* Bug Fix: `:s3_protocol` can now be defined as either String or Symbol
* Bug Fix: Tempfiles are now rewinded before get passed into `after_flush_writes`
* Feature: Added expiring_url method to Fog Storage
* API CHANGE: Paperclip now tested against AWS::SDK 1.5.2 onward
* Bug Fix: Improved the output of the content_type validator so the actual failure is displayed
* Feature: Animated formats now identified using ImageMagick.
* Feature: AttachmentAdapter now support fetching attachment with specific style.
* Feature: Paperclip default options can now be configured in Rails.configuration.
* Feature: add Geometry#resize_to to calculate dimensions of new source.
* Bug Fix: Fixed a bug whereby a file type with multiple mime types but no official type would cause
  the best_content_type to throw an error on trying nil.content_type.
* Bug Fix: Fix problem when the gem cannot be installed on the system that has Asepsis installed.

3.0.4:

* Feature: Adds support for S3 scheme-less URL generation.

3.0.3:

* Bug Fix: ThumbnailProcessor now correctly detects and preserve animated GIF.
* Bug Fix: File extension is now preserved in generated Tempfile from adapter.
* Bug Fix: Uploading file with unicode file name now won't raise an error when
  logging in the AWS is turned on.
* Bug Fix: Task "paperclip:refresh:missing_styles" now work correctly.
* Bug Fix: Handle the case when :restricted_characters is nil.
* Bug Fix: Don't delete all the existing styles if we reprocess.
* Bug Fix: Content type is now ensured to not having a new line character.
* API CHANGE: Non-Rails usage should include Paperclip::Glue directly.

  `Paperclip::Railtie` was intended to be used with Ruby on Rails only. If you're
  using Paperclip without Rails, you should include `Paperclip::Glue` into
  `ActiveRecord::Base` instead of requiring `paperclip/railtie`:

      ActiveRecord::Base.send :include, Paperclip::Glue

* Bug Fix: AttachmentContentTypeValidator now allow you to specify :allow_blank/:allow_nil
* Bug Fix: Make sure content type always a String.
* Bug Fix: Fix attachment.reprocess! when using storage providers fog and s3.
* Bug Fix: Fix a problem with incorrect content_type detected with 'file' command for an empty file on Mac.

3.0.2:

* API CHANGE: Generated migration class name is now plural (AddAttachmentToUsers instead of AddAttachmentToUser)
* API CHANGE: Remove Rails plugin initialization code.
* API CHANGE: Explicitly require Ruby 1.9.2 in the Gemfile.
* Bug Fix: Fixes AWS::S3::Errors::RequestTimeout on Model#save.
* Bug Fix: Fix a problem when there's no logger specified.
* Bug Fix: Fix a problem when attaching Rack::Test::UploadedFile instance.

3.0.1:

* Feature: Introduce Paperlip IO adapter.
* Bug Fix: Regression in AttachmentContentTypeValidator has been fixed.
* API CHANGE: #to_file has been removed. Use the #copy_to_local_file method instead.

3.0.0:

* API CHANGE: Paperclip now requires at least Ruby on Rails version 3.0.0
* API CHANGE: The default :url and :path have changed. The new scheme avoids
  filesystem conflicts and scales to handle larger numbers of uploads.

  The easiest way to upgrade is to add an explicit :url and :path to your
  has_attached_file calls:

      has_attached_file :avatar,
        :path => ":rails_root/public/system/:attachment/:id/:style/:filename",
        :url => "/system/:attachment/:id/:style/:filename"

* Feature: Adding Rails 3 style validators, and adding `validates_attachment` method as a shorthand.
* Bug Fix: Paperclip's rake tasks now loading records in batch.
* Bug Fix: Attachment style name with leading number now not raising an error.
* Bug Fix: File given to S3 and Fog storage will now be rewinded after flush_write.
* Feature: You can now pass addional parameter to S3 expiring URL, such as :content_type.

2.7.0:

* Bug Fix: Checking the existence of a file on S3 handles all AWS errors.
* Bug Fix: Clear the fingerprint when removing an attachment.
* Bug Fix: Attachment size validation message reads more nicely now.
* Feature: Style names can be either symbols or strings.
* Compatibility: Support for ActiveSupport < 2.3.12.
* Compatibility: Support for Rails 3.2.

2.6.0:

* Bug Fix: Files are re-wound after reading.
* Feature: Remove Rails dependency from specs that need Paperclip.
* Feature: Validation matchers support conditionals.

2.5.2:

* Bug Fix: Can be installed on Windows.
* Feature: The Fog bucket name, authentication, and host can be determined at runtime via Proc.
* Feature: Special characters are replaced with underscores in #url and #path.

2.5.1:

* Feature: After we've computed the content type, pass it to Fog.
* Feature: S3 encryption with the new :s3_server_side_encryption option.
* Feature: Works without ActiveRecord, allowing for e.g. mongo backends.

2.5.0:

* Performance: Only connect to S3 when absolutely needed.
* Bug Fix: STI with cached classes respect new options.
* Bug Fix: conditional validations broke, and now work again.
* Feature: URL generation is now parameterized and can be changed with plugins or custom code.
* Feature: :convert_options and :source_file_options to control the ImageMagick processing.
* Performance: String geometry specifications now parse more quickly.
* Bug Fix: Handle files with question marks in the filename.
* Bug Fix: Don't raise an error when generating an expiring URL on an unassigned attachment.
* Bug Fix: The rake task runs over all instances of an ActiveRecord model, ignoring default scopes.
* Feature: DB migration has_attached_file and drop_attached_file methods.
* Bug Fix: Switch from AWS::S3 to AWS::SDK for the S3 backend.
* Bug Fix: URL generator uses '?' in the URL unless it already appears and there is no prior '='.
* Bug Fix: Always convert the content type to a string before stripping blanks.
* Feature: The :keep_old_files option preserves the files in storage even when the attachment is cleared or changed.
* Performance: Optimize Fog's public_url access by avoiding it when possible.
* Bug Fix: Avoid a runtime error when generating the ID partition for an unsaved attachment.
* Performance: Do not calculate the fingerprint if it is never persisted.
* Bug Fix: Process the :original style before all others, in case of a dependency.
* Feature: S3 headers can be set at runtime by passing a proc object as the value.
* Bug Fix: Generating missing attachment styles for a model which has had its attachment changed should not raise.
* Bug Fix: Do not collide with the built-in Ruby hashing method.