Álvaro Jimeno 

Maybe it's not an ASS file?  Can you share it?

By pure coincidence, or maybe because I had had coffee just before, I saw that the Project Settings had a 'There are additional file types installed' message, and when clicking on it you could enable ASS file types again. Now it works. Hope this helps if someone else runs into this issue.

 Álvaro Jimeno 

Worth discussing how this can happen as this will help with your advice.

If you add this new filetype, or make any changes to the default settings used for creating projects, existing projects will not be affected. This is important and deliberate.

If you use project templates to create projects these will also not be affected by changes you make to your default settings. So you will have to apply these changes to each template you intend to use wit’s a new filetype or other edit going forward.  Similarly important and deliberate behavior.