title: Django ImageField widget to add thumbnail with clearable checkbox
body: <p>I have a model like this</p>
  
  <pre><code>class Participation(models.Model):
      # ... some attributes ...
      thumbnail = models.ImageField(
          verbose_name='immagine di copertina',
          blank=True,
          null=True,
      )
  </code></pre>
  
  <p>and I add the following code to show a thumbnail in the admin page:</p>
  
  <pre><code>class AdminImageWidget(ClearableFileInput):
  """A ImageField Widget for admin that shows a thumbnail"""
  
  def render(self, name, value, attrs=None):
      output = []
      if value and getattr(value, 'url', None):
          image_url = value.url
          file_name = str(value)
          output.append(
              u'%s&amp;nbsp;'
              u'&lt;a href="%s" target="_blank"&gt;'
              u'&lt;img src="%s" alt="%s" style="max-width: 100px; max-height: 100px; border-radius: 5px;" /&gt;'
              u'&lt;/a&gt;&lt;br/&gt;&lt;br/&gt;%s '
              % (_('Currently:'), image_url, image_url, file_name, _('Change:'))
          )
      output.append(super(ClearableFileInput, self).render(name, value, attrs))
      return mark_safe(u''.join(output))
  </code></pre>
  
  <p>Even if it worked fine for the thumbnail, the <code>super(ClearableFileInput, self).render(name, value, attrs)</code> calling is adding just the <em>browse</em> button, but it isn't showing the clear checkbox, so I cannot delete the selected thumbnail.</p>
  
  <p>What's wrong with this code? How can I add the checkbox mantaining the thumbnail too?</p>