https://github.com/nostr-protocol/nips/blob/master/96.md

nostr.build does not implement it, nor does anyone i know of

i'd be glad to be contradicted but i have a nostr.build subscription and the only way i put data on it that gets attributed to me is through their webapp, not had a single image upload via clients go to them even though all clients use nostr.build, practically, and my client is signing events of course

so, maybe i'm misunderstanding this shit but it seems to me like either the clients are incompetent and/or nostr.build doesn't implement nip96 properly