how allow '.' for an excel file name when saving as using late binding

I just came up with this issue, we have a library that uses Reflection for manipulate excel files. And when a user is trying to save a file that has a '.' in the file name (eg: 01.02.xls) it won'd include ex .xls in the saved file (result: 01.02).

workbook.GetType().InvokeMember("SaveAs", BindingFlags.InvokeMethod, null, workbook, new Object[] { filepath.Remove(filepath.LastIndexOf(".")), 56 });

Is there an option that allows '.' inside the file name?

Thanks in advance