从远程链接下载

     /// <summary>
        /// 从外部链接下载附件
        /// </summary>
        /// <param name="id"></param>
        /// <returns></returns>
        public ActionResult DownloadFromExternalLink(string url)
        {
            //string uploadPath = System.Configuration.ConfigurationManager.AppSettings["BPMAttachments"];
            //string dirRelativePath = "externalLinks";
            //string dirAbsolutePath = uploadPath + dirRelativePath;
            string fileName = Path.GetFileName(url);
            WebClient client = new WebClient();
            //client.DownloadFile(url, dirAbsolutePath + fileName);
            var fileData = client.DownloadData(url);
            var contentType = MimeMapping.GetMimeMapping(fileName);
            HttpContext.Response.AddHeader("content-disposition", "inline;filename=" + fileName);
            return File(fileData, contentType);
        }