string fileName = Request.PhysicalApplicationPath + 文件地址;
        System.IO.FileInfo file = new System.IO.FileInfo(fileName);

        UploadFile uploadFile = new UploadFile();
        Response.AddHeader("Content-Disposition", "attachment; filename=" + HttpUtility.UrlEncode(uploadFile.GetSAttachName(file.Name), Encoding.UTF8));
        Response.AddHeader("Content-Length", file.Length.ToString());
        Response.ContentType = "application/octet-stream";
        Response.WriteFile(file.FullName);

------解决方案--------------------
把服务器IIS的meta设置的txt映射项删除，这样浏览器就只能下载txt，而不能打开txt了。