There’s a bunch of nip-96 platforms like nostr.build, so they definitely can be uploaded to other services. https://github.com/aljazceru/awesome-nostr#nip-96-file-storage-servers

The nip-94 events are immutable. I think relays will eventually end up deleting events with dead links. 

 You could upload to several servers first and then publish the nip94 event with several urls.