File upload

File upload is a 'virtual' field type not meant to be used directly, but through vendor implementations (Paperclip/Dragonfly/Carrierwave)

Those implementation share common characteristics.

If defaults don't fit, fine-tune with:

field :asset do
  # set a method available to your asset (defaults to :thumb, :thumbnail or '100x100>' for Dragonfly)
  thumb_method :large
  
  # for delete checkbox in forms
  delete_method :asset_delete     # don't forget to whitelist if you use :attr_accessible
  
  # in case of a validation failure, to retain asset in the form (not available for Paperclip)
  cache_method :asset_cache       # don't forget to whitelist if you use :attr_accessible
end

More here