Decouple the Interpolations class from the Attachment classs, you can…

… extend Paperclip with custom interpolations as paperclip-extended does.

lib/paperclip/attachment.rb

@@ -27,12 +27,33 @@ def self.default_options
       }
     end
 
-    attr_reader :name, :instance, :default_style, :convert_options, :queued_for_write, :whiny, :options, :source_file_options
+    attr_reader :name, :instance, :default_style, :convert_options, :queued_for_write, :whiny, :options, :source_file_options, :interpolator
     attr_accessor :post_processing
 
     # Creates an Attachment object. +name+ is the name of the attachment,
     # +instance+ is the ActiveRecord object instance it's attached to, and
     # +options+ is the same as the hash passed to +has_attached_file+.
+    #
+    # Options include:
+    #
+    #  +url+ - a relative URL of the attachment. This is interpolated using +interpolator+
+    #  +path+ - where on the filesystem to store the attachment. This is interpolated using +interpolator+
+    #  +styles+ - a hash of options for processing the attachment. See +has_attached_file+ for the details
+    #  +only_process+ - style args to be run through the post-processor. This defaults to the empty list
+    #  +default_url+ - a URL for the missing image
+    #  +default_style+ - the style to use when don't specify an argument to e.g. #url, #path
+    #  +storage+ - the storage mechanism. Defaults to :filesystem
+    #  +use_timestamp+ - whether to append an anti-caching timestamp to image URLs. Defaults to true
+    #  +whiny+, +whiny_thumbnails+ - whether to raise when thumbnailing fails
+    #  +use_default_time_zone+ - related to +use_timestamp+. Defaults to true
+    #  +hash_digest+ - a string representing a class that will be used to hash URLs for obfuscation
+    #  +hash_data+ - the relative URL for the hash data. This is interpolated using +interpolator+
+    #  +hash_secret+ - a secret passed to the +hash_digest+
+    #  +convert_options+ - flags passed to the +convert+ command for processing
+    #  +source_file_options+ - flags passed to the +convert+ command that controls how the file is read
+    #  +processors+ - classes that transform the attachment. Defaults to [:thumbnail]
+    #  +preserve_files+ - whether to keep files on the filesystem when deleting to clearing the attachment. Defaults to false
+    #  +interpolator+ - the object used to interpolate filenames and URLs. Defaults to Paperclip::Interpolations
     def initialize name, instance, options = {}
       @name              = name
       @instance          = instance
@@ -65,6 +86,7 @@ def initialize name, instance, options = {}
       @queued_for_write      = {}
       @errors                = {}
       @dirty                 = false
+      @interpolator          = (options[:interpolator] || Paperclip::Interpolations)
 
       initialize_storage
     end
@@ -138,7 +160,7 @@ def url(style_name = default_style, use_timestamp = @use_timestamp)
     # file is stored in the filesystem the path refers to the path of the file
     # on disk. If the file is stored in S3, the path is the "key" part of the
     # URL, and the :bucket option refers to the S3 bucket.
-    def path style_name = default_style
+    def path(style_name = default_style)
       original_filename.nil? ? nil : interpolate(@path, style_name)
     end
 
@@ -377,8 +399,8 @@ def post_process_styles(*style_args) #:nodoc:
       end
     end
 
-    def interpolate pattern, style_name = default_style #:nodoc:
-      Paperclip::Interpolations.interpolate(pattern, self, style_name)
+    def interpolate(pattern, style_name = default_style) #:nodoc:
+      interpolator.interpolate(pattern, self, style_name)
     end
 
     def queue_existing_for_delete #:nodoc:

lib/paperclip/thumbnail.rb

@@ -17,7 +17,7 @@ class Thumbnail < Processor
     #
     # Options include:
     #
-    #   +geometry+ - the desired width and height of the thumbnail
+    #   +geometry+ - the desired width and height of the thumbnail (required)
     #   +file_geometry_parser+ - an object with a method named +from_file+ that takes an image file and produces its geometry and a +transformation_to+. Defaults to Paperclip::Geometry
     #   +string_geometry_parser+ - an object with a method named +parse+ that takes a string and produces an object with +width+, +height+, and +to_s+ accessors. Defaults to Paperclip::Geometry
     #   +source_file_options+ - flags passed to the +convert+ command that influence how the source file is read

test/attachment_test.rb

@@ -1039,27 +1039,41 @@ def do_after_all; end
       end
     end
   end
-  
+
   context "an attachment with delete_file option set to false" do
     setup do
       rebuild_model :preserve_files => true
       @dummy = Dummy.new
       @file = File.new(File.join(File.dirname(__FILE__), "fixtures", "5k.png"), 'rb')
       @dummy.avatar = @file
       @dummy.save!
-      @attachment = @dummy.avatar 
-      @path = @attachment.path      
+      @attachment = @dummy.avatar
+      @path = @attachment.path
     end
-    
-    should "not delete the files from storage when attachment is destroyed" do      
+
+    should "not delete the files from storage when attachment is destroyed" do
       @attachment.destroy
       assert File.exists?(@path)
     end
-    
-    should "not dleete the file when model is destroy" do
+
+    should "not delete the file when model is destroy" do
       @dummy.destroy
       assert File.exists?(@path)
     end
   end
 
+  context "setting an interpolation class" do
+    should "produce the URL with the given interpolations" do
+      Interpolator = Class.new do
+        def self.interpolate(pattern, attachment, style_name)
+          "hello"
+        end
+      end
+
+      instance = Dummy.new
+      attachment = Paperclip::Attachment.new(:avatar, instance, :interpolator => Interpolator)
+
+      assert_equal "hello", attachment.url
+    end
+  end
 end