Please, try setting the SaveFileDialog in the following manner :
SaveFileDialog dialog = new SaveFileDialog();
dialog.DefaultExt = "*.xls";
dialog.Filter = "Files(*.xls)|*.xls";
If the issue persists, it would be very helpful for us if you're able to zip and send us a sample application, demonstrating this issue, so that we can debug it locally and provide further support.
All the best,
the Telerik team