I thought .nc was the default output, but then it's probably what is set in the post processors I happen to use.

If the post processor file you're using is set to tap, and everything else is ok, open the post processor.
Either from within F360 - manufacture -> 'manage' drop down menu -> Post Library, then in the new window right-click on the post, and select Edit, or alternatively find).
or in the local storage folder - C:\Users\<user>\AppData\Roaming\Autodesk\Fusion 360 CAM\Posts

You need to change the extension = "nc"; variable (it's near the top).
Then save the file, and it should be updated next time you post code in F360.