OMEMO is the (one of the) encryption thing(s) for XMPP, as any thing for encryption, you have to verify the other person’s you’re talking to keys, most client verify new keys automatically, and no one really cares. This means that if a fed (or anyone) breaks into your friend’s account, you’d think that you’re talking to your friend, with encryption, but it’s a fed, your client just signed all the new keys by default
thankfully, new clients are adding some “verified” keys, and it will warn you when a message was send from an untrusted key, or you’ll simply won’t recieve messages from untrusted keys
When you upload something using http_upload using omemo, the file is encrypted, but using AES symmetric cryptography, rather than recipient’s public key and your public key. As it is symmetric cryptography the passphrase is stored somewhere else, in this case, it is stored in the URI. Meaning that if an attacker could get the URI you’ve sent, they could get the file.
Obviously, this is not likely to happen because the message with the URI is encrypted with TLS and OMEMO, but I think the file should’ve be encrypted with recipient’s public key.
The reason of this (i think) is that so you can use another tools to download omemo files. Like omemo-wget.
Anyways, if you want to share files and get encrypted on-the-go you can always use pgp.