Feature request: Add support for Dragonfly's ability to retain file uploads across form redisplays

[sauy7]

From Dragonfly docs:

Retaining across form redisplays

When a model fails validation, you don't normally want to have to upload your attachment again, so you can avoid having to do this by including a hidden field in your form retained_<attribute_name>, e.g.

<% form_for @photo, :html => {:multipart => true} do |f| %>
  ...
  <%= f.file_field :image %>
  <%= f.hidden_field :retained_image %>
  ...
<% end %>