Encrypting data - rails + attachment_fu

Question

We've been using attachment_fu to upload files into our application. We now have to be able to encrypt uploaded data using AES. We use Rails 2.3.8 and Ruby 1.8.7. Attachment_fu is being used to check MIMEtypes, so we can't encrypt with the before_save callback (or the MIMEtype will get corrupted) and attachment_fu uses temporary files, so I'm worried about co-opting attachment_fu's final save_to_storage method, because the temp files will be stored in plaintext before save occurs.