// file name value of inputfile control
string file = fileUploadTextBox.Value;
// get extension
string ext = file.Substring(file.IndexOf("."),file.Length-file.IndexOf("."));
// ad your new file name
string filename = "MyCustomText" + ext;
// get the path to the file
string fullPath = System.Web.HttpContext.Current.Server.MapPath("C:\mydirectory" + filename)
// upload!
fileUploadTextBox.PostedFile.SaveAs(fullPath);