Microsoft 365 Apps for Windows 10:Default file format

Hi, when deploying M365 Apps via Intune it is mandatory to define the "Default file format". Either "Office Open Document Format" or "Office Open XML Format". This selection leads to the situation, that when saving an excel worksheet one of the above file formats are are set as the default file format to save as. How would I be able to let e.g. xlms be the default file format? 

cheers, heinzelrumpel