Thomas Steiner:

Could file_handler be dynamically extended, without pushing a new manifest? Conceived-ish example: you have a freeium app that initially handles just .foo files, you install it, love it, unlock the premium version, and now it can also handle .foopremium files.

Good thought. I think we should add that as something to explore for V2. The same idea has been raised for the proposed Shortcuts API and it's also relevant for web share target - ideally these "handler" APIs should have declarative and procedural versions