void Uploader_FileUploaded(object sender, UploaderEventArgs args)
 {
  string folder = Server.MapPath("~/MyFiles");
  string savepath = System.IO.Path.Combine(folder, args.FileName);
  args.MoveTo(savepath);

  //because the args.FileName is the origin name, so now it has the origin file extension

  //now you can convert the file :

  MyApp.ConvertAVI(savepath);

  //or you can also put it into queue,and convert it later at background(so the client can know the file have been uploaded and do not need wait conversation.)

  //MyApp.AddFileIntoQueue(savepath);
 }