номер = номер архива + расширение (ejemplo.txt)
public void DownloadFile(string ubicacion, string nombre)
{
Response.Clear();
Response.ContentType = @"application\octet-stream";
System.IO.FileInfo file = new System.IO.FileInfo(ubicacion);
Response.AddHeader("Content-Disposition", "attachment; filename=" + nombre);
Response.AddHeader("Content-Length", file.Length.ToString());
Response.ContentType = "application/octet-stream";
Response.WriteFile(file.FullName);
Response.Flush();
}