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)
Sets a `thumbnail` attribute on the image field class.
        On thumbnail you can access name, url, path attributes