foreach (EO.Web.AJAXPostedFile file in Uploader1.PostedFiles)
{
    //Get the original file name. Note:
    //1. This returns the full file name. For example, if your
    //   FinalFileLocation is set to "c:\upload", this returns
    //   "c:\upload\abc.jpg" instead of "abc.jpg";
    //2. FinalFileName is valid only when FinalFileLocation is
    //   set, when FinalFileLocation is not set, you should use
    //   TempFileName instead
    string fileName = file.FinalFileName;

    //Split the full file name into directory and file name
    string dir = System.IO.Path.GetDirectoryName(fileName);
    string name = System.IO.Path.GetFileName(fileName);

    //Change the file name
    name = "TomC_" + name;

    //Create the new file name
    string newFileName = System.IO.Path.Combine(dir, name);

    //Rename the file
    System.IO.File.Move(fileName, newFileName);
}