Validere format i image upload

God formiddag alle eksperter. Jeg er i min skole ved at lave et Billedeupload som også skalere billedet. Indtil videre virker det perfekt.
Men den kan bare ikke li' at jeg uploader andet end JPeg :(

public partial class _Default : System.Web.UI.Page
{
    private string savepath = "~/Images/";
    private string original = HttpContext.Current.Request.PhysicalApplicationPath + "/Images/";
    private string thumb = HttpContext.Current.Request.PhysicalApplicationPath + "/Images/Thumb/";


    protected void Page_Load(object sender, EventArgs e)
    { }

    private System.Drawing.Image Scale(System.Drawing.Image image, int maxSizeWidthOrHeigh)
    {
        int w = image.Width;
        int h = image.Height;

        if (w > h)
        {
            w = maxSizeWidthOrHeigh;
            h = image.Height * maxSizeWidthOrHeigh / image.Width;
        }
        else
        {
            w = image.Width * maxSizeWidthOrHeigh / image.Height;
            h = maxSizeWidthOrHeigh;
        }

        using (Bitmap bm = new Bitmap(w, h))
        {
            using (Graphics gr = Graphics.FromImage((System.Drawing.Image)bm))
            {
                gr.InterpolationMode = InterpolationMode.HighQualityBicubic;
                gr.DrawImage(image, 0, 0, w, h);
            }
            try
            {
                return (System.Drawing.Image)bm.Clone();
            }
            finally
            {
                bm.Dispose();
            }
        }
    }


    private void saveAsThumbnail(string original, string thumb, int maxSize)
    {
        System.Drawing.Image originalImg = System.Drawing.Image.FromFile(original);

        using (System.Drawing.Image thumbImg = (System.Drawing.Image)Scale(originalImg, maxSize))
        {
            thumbImg.Save(thumb, ImageFormat.Jpeg);
            thumbImg.Dispose();
            originalImg.Dispose();
        }
    }

    protected void Button_upload_Click(object sender, EventArgs e)
    {
        if (FileUpload_default.HasFile)
        {
            string filename = FileUpload_default.FileName;
            string savename = Server.MapPath(savepath) + filename;



            FileUpload_default.SaveAs(Request.PhysicalApplicationPath + "/Images/" + FileUpload_default.FileName);
            saveAsThumbnail(Request.PhysicalApplicationPath + "/Images/" + FileUpload_default.FileName, Request.PhysicalApplicationPath + "/Images/Thumb/" + FileUpload_default.FileName, Convert.ToInt32(textSize.Text));

            LinkThumb.Text = "Se Thumb";
            LinkThumb.NavigateUrl = "~/Images/Thumb/" + filename;

            LinkImage.Text = "Se Image";
            LinkImage.NavigateUrl = "~/Images/" + filename;

        }
        else
        {
            Response.Write("Vedhæft en fil for at fortage et upload!");
        }
}