The Save commands default file format must be configured.

When users create new Excel files, Excel 2013 saves them in the new *.xlsx format. Ensure this setting is enabled to specify all new files are created in Excel 2013. If a new file is created in an earlier format, some users may not be able to open or use the file, or they may choose a format that is less secure than the Excel 2013 format. Users can still select a specific format when they save files, but they cannot change the default of this setting from the "Excel Options" dialog box. This enforced user behavior ensures any change to the file format requires additional deliberate user interaction.