We don't need to check if something is included in Set.

As values in set are strings - check is unnecessary.

lib/paperclip.rb

@@ -318,7 +318,7 @@ module Paperclip
       end
 
       attachment_definitions[name] = {:validations => []}.merge(options)
-      Paperclip.classes_with_attachments << self.name unless Paperclip.classes_with_attachments.include?(self.name)
+      Paperclip.classes_with_attachments << self.name
       Paperclip.check_for_url_clash(name,attachment_definitions[name][:url],self.name)
 
       after_save :save_attached_files