[Zubersoft]

I just installed WhatsApp, sent a file, tapped that file and immediately saw the option to open it in MobileSheets. So I have no idea why you are unable to open the file in MobileSheets. It might be worth backing up your library in MobileSheets under Settings->Backup and Restore->Backup Library, uninstall the app, reinstall it, maybe uninstall and reinstall WhatsApp and see if that fixes the issue. As far as I can see, it's working as I would expect.

Mike