:call-seq:
  maybe_convert_extension(ext) -> string

Given a MIME type or file extension, return either a MIME type or, if
none is found, the input unchanged.

    >> maybe_convert_extension('json')
    => 'application/json'

    >> maybe_convert_extension('unknown')
    => 'unknown'

    >> maybe_convert_extension('application/xml')
    => 'application/xml'

[Parameters:]

  - ext (String)