Set default file type to xlsx when saving a spreadsheet generated from stream

this.radSpreadsheet1.SpreadsheetElement.ActiveSheetEditor.UICommandExecuting += ActiveSheetEditor_UICommandExecuting;
private void ActiveSheetEditor_UICommandExecuting(object sender, Telerik.WinForms.Controls.Spreadsheet.Commands.UICommandExecutingEventArgs e)
{
    if (e.Command == this.radSpreadsheet1.SpreadsheetElement.CommandDescriptors.SaveFile.Command)
    {
        e.Cancel();
        string desktopPath = Environment.GetFolderPath(Environment.SpecialFolder.Desktop);
        var dlg = new System.Windows.Forms.SaveFileDialog();
        dlg.Filter = "Xlsx Files (*.xlsx)|*.xlsx";
        dlg.DefaultExt = "xlsx";
        dlg.FileName = "Test.xlsx";
        dlg.AddExtension = true;
        dlg.RestoreDirectory = true;
        dlg.InitialDirectory = desktopPath;
        if (dlg.ShowDialog() == DialogResult.OK)
        {
            var formatProvider = new XlsxFormatProvider();
            using (Stream output = new FileStream(dlg.FileName, FileMode.Create))
            {
                formatProvider.Export(this.radSpreadsheetRibbonBar1.AssociatedSpreadsheet.Workbook, output);
            }
        }
    }
}