file servers can define their required pow per byte values for upload and retrieval in nip96.json

for attachments, each alturl tag could define required pow for retrieval

resource constrained clients could prefer low pow for retrieval file servers

also requesting lower quality version of media file would require lower pow per retrieval

however pow per retrieval and or upload could be relative to fileserver load. in this case, if its dynamic, maybe nip96.json is not best place to define it, nor event tags