By default, the Excel 2000-2003 File Type recognizes the .xls and .xlt file extensions.
You can configure some aspects of SDL file types in Management > Linguistic Tool Setup > File Types.
Note: To process Microsoft Excel 2000-2003 files, Microsoft Excel 2007 (or Microsoft Excel 2003 with the Office 2007 compatibility pack) must be installed on the machine that runs the FTS server.