There have been a few proposals, such as putting the files on nostr in base32, PR 751 media attachments, etc.

I think your comment is important. I don't like the single-URL references. Because events can't easily be censored (we can copy them elsewhere and nothing breaks) but URLs can. It must be that the file reference doesn't break if the files have to be moved.

I suggest using a SHA256 hash of the file as the file reference, with an indication of some sort of distributed storage (DHT, IPFS, etc) which should be flexible so that alternate distributed storage schemes can be added over time. The hash of the file will never become a broken link.