how to rename file when upload (change file name when upload)

  asp.net FileUpload example: how to rename file when upload (change file name when upload) 
string extension = Path.GetExtension(FileUpload1.PostedFile.FileName);
FileUpload1.SaveAs(@"C:\Test\" + FileUpload1.FileName.Replace(FileUpload1.FileName, "Mandar" + extension));

Tip : To use Path.GetExtension() method ,Add namespace System.IO namespace at the top