title: Cannot upload a file in activeadmin
body: <p>I have a basic file upload with carrierwave:</p>
  
  <p><strong>uploaders/image_uploader.rb:</strong></p>
  
  <pre><code>class ImageUploader &lt; CarrierWave::Uploader::Base
    include CarrierWave::RMagick
    storage :file
    def store_dir
      "uploads/#{model.class.to_s.underscore}/#{mounted_as}/#{model.id}"
    end
  end
  </code></pre>
  
  <p><strong>models/event.rb:</strong></p>
  
  <pre><code>class Event &lt; ActiveRecord::Base
    ...
    mount_uploader :picture, ImageUploader
  end
  </code></pre>
  
  <p>I can upload an image with a form in the app (not the admin part) and it works fine. I see the image path in the <code>picture</code> field in ActiveAdmin.</p>
  
  <p>But I cannot change the picture field directly in ActiveAdmin. I tried to create a new event with an image, or change an existing event. If an image was present, it stayed the same. If there were no image previously, no image were added. In short, I can view the <code>picture</code> field in ActiveAdmin, but I cannot change it. This is the case only for the <code>picture</code> field (I can change the other fields)</p>
  
  <p>Note: No upload starts when I submit the form (I don't see the uploading xx% in the bottom left of chrome). I also tried other browers, same behaviour. I get no errors in the rails server console.</p>
  
  <p><strong>admin/events.rb:</strong></p>
  
  <pre><code>ActiveAdmin.register Event do
    permit_params :user_id, :category_id, :name, :date, :description, :text, :picture
  
   # tried adding this, form still works, but image still doesn't work.
   form multipart: true do |f|
      f.inputs "Event details" do
        f.input :category_id
        f.input :name
        f.input :description
        f.input :picture, as: :file
      end
      f.actions
    end
  end
  </code></pre>
  
  <p>Why can't I change the <code>picture</code> field in ActiveAdmin ?</p>