- attachment setting was crashing when `item_elt` was not specified due to argument typo
- if `signer` is set to None in attachment data, client's bare jid will be used to sign
the item
rel 381
- this is based on a protoXEP, not yet an official XEP: https://github.com/xsf/xeps/pull/1228
- XEP-0470: `set` attachment handler can now be async
rel 381