Try this piece of code:

    private string UploadFile()
    {
        Random rand = new Random();
        //string imagepath;
        int guid = rand.Next();
        string ext = Path.GetExtension(FileUpload.FileName);
        string Serverpath;
        Serverpath = "~/Upload/" + guid.ToString() + ext.ToString();


        //string filepath = MapPath(Serverpath).ToString();
        FileUpload.PostedFile.SaveAs(MapPath(Serverpath));
        return Serverpath;
    }