Refactored: :image and :file dragonfly apps, proper helper method definition, form multipart

- dragonfly attachments are now served using two differently configured apps: :image and :file
  :image serves image files, while :file serves generic files with default behaviour to download
  file (Content-Disposition: attachment) instead of showing it
- #description helper moved to dragonfly configuration block
- using file fields in form builder automatically sets multipart=true for the form