The new default :path and :include include the name of the model and
also nests the model ID under a series of subdirectories, improving
filesystem access speed when more than 1024 models have saved
attachments.

The easiest way to upgrade is to add an explicit :url and :path to your
has_attached_file calls:

    has_attached_file :avatar,
      :path => ":rails_root/public/system/:attachment/:id/:style/:filename",
      :url => "/system/:attachment/:id/:style/:filename"