public function filter_attachment_url($content, $post_id)
 {
     if (is_admin()) {
         //No need to filter on the admin side
         return $content;
     }
     $acl = SwpmAccessControl::get_instance();
     if (has_post_thumbnail($post_id)) {
         return $content;
     }
     if ($acl->can_i_read_post($post_id)) {
         return $content;
     }
     return SwpmUtils::get_restricted_image_url();
 }