/// <summary>
        /// 获取有效客户端文件控件集合,文件控件必须上传了内容，为空将被忽略,
        /// 注意:Form标记必须加入属性 enctype="multipart/form-data",服务器端才能获取客户端file控件.
        /// </summary>
        public static List<HttpPostedFile> GetFileControls()
        {
            var result = new List<HttpPostedFile>();
            var files = HttpContext.Current.Request.Files;
            if (files.Count == 0)
                return result;
            for (int i = 0; i < files.Count; i++)
            {
                var file = files[i];
                if (file.ContentLength == 0)
                    continue;
                result.Add(files[i]);
            }
            return result;
        }