title: sorl-thumb doesn't make thumbnails
body: <p>I don't know what I'm doing wrong, but sorl-thumb just uploads the image but doesn't make thumbs...</p>
  
  <p>model.py</p>
  
  <pre><code> from sorl.thumbnail.fields import ImageWithThumbnailsField ,ThumbnailField
  
   imagen = ImageWithThumbnailsField(upload_to='images',
                                        thumbnail={'size': (75, 75)},
                                        extra_thumbnails={'icon': {'size': (25, 25),'options': ['crop', 'upscale']},
                                                          'large': {'size': (200, 400)},
                                                          },
                                      )
  </code></pre>
  
  <p>setting</p>
  
  <pre><code>'sorl.thumbnail',
  </code></pre>
  
  <p>What more do I need? I've followed the documentation.</p>