docs: def _set_thumbnail(self, instance=None, **kwargs):
          """
          Sets a `thumbnail` attribute on the image field class.
          On thumbnail you can access name, url, path attributes
          """
          image_field = getattr(instance, self.name)
          if image_field:
              thumbnail_filename = _get_thumbnail_filename(image_field.name)
  
              thumbnail_field = ThumbnailField(thumbnail_filename, self.storage)
              setattr(image_field, 'thumbnail', thumbnail_field)