mark instance as changed after assigning new file

lib/paperclip/attachment.rb

@@ -110,6 +110,8 @@ module Paperclip
       # Reset the file size if the original file was reprocessed.
       instance_write(:file_size,   @queued_for_write[:original].size)
       instance_write(:fingerprint, @queued_for_write[:original].fingerprint) if instance_respond_to?(:fingerprint)
+      updater = :"#{name}_file_name_will_change!"
+      instance.send updater if instance.respond_to? updater
     end
 
     # Returns the public URL of the attachment with a given style. This does

test/attachment_test.rb

@@ -1207,6 +1207,13 @@ class AttachmentTest < Test::Unit::TestCase
         assert_equal creation.to_i, @dummy.avatar.created_at
         assert_not_equal now.to_i, @dummy.avatar.created_at
       end
+
+      should "set changed? to true on attachment assignment" do
+        @dummy.avatar = @file
+        @dummy.save!
+        @dummy.avatar = @file
+        assert @dummy.changed?
+      end
     end
 
     context "and avatar_updated_at column" do