Image file input with Formtastic and ActiveAdmin

I started using formstatic but I need to make a file field with an image preview. I mean, when I edit an object, I want to see the image already linked.

How can I do that?

The answer is to use the hint attribute :

ActiveAdmin.register Event do
  form :html => { :enctype => "multipart/form-data" } do |f|
    f.input :map, :as => :file, :hint => f.template.image_tag(f.object.map.url(:thumb))
  end
end

Bye