# File test/testhelper.rb, line 27
def get_file_from_url(url,expected_file_filename)
        assert(FileSystemImage.is_file_system_image_filename?(url),"#{url} should have a known file system image extension")
        image=@@filecache.get_file_system_image(url)    
        raise "no native files identified in image" if image.files.nil? || image.files.length==0
        raise "image.files should of type FileContainer but was #{image.files.class} (file system=#{image.file_system})" unless image.files.kind_of?(FileContainer)
        native_file=image.files[expected_file_filename]
        assert_not_nil(native_file,"#{expected_file_filename} should exist in #{url} - files found: #{image.catalog}")
        native_file    
end