Set it to `inline` because the vast majority of what's sent is multimedia
content while `attachment` would have the side-effect of triggering a
download dialog.
Closes: https://git.pleroma.social/pleroma/pleroma/-/issues/3114
In general, tests that match these criteria can be made async:
- Doesn't use real Cachex.
- Doesn't write to the Config / Application Environment.
- Uses Mock. Using Mox is fine.
- Uses the streamer.
