title: Empty folders when file is deleted using CarrierWave
body: <p>When I delete an uploaded file using CarrierWave and Mongoid it remove the file but leave emty folders.</p>
  
  <pre><code>/files/:user_id/images/:file_id/ ['image.png', 'content_image.png', 'thumb_image.png']
  </code></pre>
  
  <p>I want the <code>:file_id</code> folder to also be removed when the file is deleted. Is there a way to do this?</p>