/// <summary>
        /// attach a file to a topic
        /// </summary>
        /// <param name="fid"></param>
        /// <param name="tid"></param>
        /// <returns></returns>
        public JsonResult AttachFileToTopic(string fid, string tid)
        {
            var msg = XFile.AttachToTopic(fid, tid);

            return(Json(msg));
        }