While going through epubcheck's tests, I noticed that it throws an error if file names have spaces in them. We previously discussed whether these should be allowed in #808

But in this revision we've changed to referencing the URL standard, so should we revist this issue again? Otherwise, it's strange for epubcheck to ban them when that specification doesn't.

I'm not suggesting we have to allow spaces now, as suddenly allowing spaces may expose interoperability issues hidden by the previous ban, but perhaps we should add a recommendation/restriction against their use to the OCF file and path naming section? It would help to have epubcheck aligned with spec requirements so it's not seen as adding arbitrary restrictions.