Modified your code.
string date = DateTime.Now.ToShortDateString();
string newdate = date.Replace("/", ".");
string filePath = Server.MapPath("~/C:/HKGOUTBOUND/" + newdate + ".TXT");
string FileName = newdate + ".TXT";
Response.ContentType = "Application/TXT";
Response.AddHeader("Content-Disposition", "attachment;filename=" + FileName + ";");
Response.TransmitFile(Server.MapPath("~/C:/HKGOUTBOUND/" + newdate + ".TXT"));
Response.Flush();
Response.End();