The default :url option is very dangerous.

lib/paperclip/attachment.rb

@@ -25,7 +25,7 @@ def self.default_options
         :source_file_options   => {},
         :storage               => :filesystem,
         :styles                => {},
-        :url                   => "/system/:attachment/:id/:style/:filename",
+        :url                   => "/system/:class/:id/:attachment/:style/:filename",
         :url_generator         => Paperclip::UrlGenerator,
         :use_default_time_zone => true,
         :use_timestamp         => true,