For your Excel file "Microsoft Excel 2.1 Worksheet", this is older Excel file format that we do not support it, so you are getting the exception. For your information, Aspose.Cells supports BIFF8 or greater formats (Excel 97 - Excel 2010). To confirm it is "Microsoft Excel 2.1 Worksheet", you may try to open it into MS Excel 2003 and try Save it As, now you can see the save as type i.e. "Microsoft Excel 2.1 Worksheet" in the Save As dialog box.
As a workaround, you may try the following steps to work with the file for Aspose.Cells product, so that Aspose.Cells could open/process the file fine, there is no alternative I am afraid:
Open your template file into MS Excel (e.g..,Ms Excel 2003 or 2007).
Save the file As "Microsoft Office Excel Workbook".
Now you can use Aspose.Cells to open/manipulate the file for your requirements, it will work fine.