string FileName = listID[i].ToString();
FileInfo file = new FileInfo(FileName);
if (file.Exists)
{
System.Web.HttpResponse response = System.Web.HttpContext.Current.Response;
response.ClearContent();
response.Clear();
response.ContentType = "text/plain";
response.AddHeader("Content-Disposition", "attachment; filename=" + file.FullName + ";");
response.TransmitFile(file.FullName);
response.Flush();
response.End();
response.AddHeader("Content-Disposition", "attachment; filename='" + file.Name + "';");
// или
// response.AddHeader("Content-Disposition", "attachment; filename=\"" + file.Name + "\";");