The newest git HEAD of MIME already knows about APNG, but this
hasn’t been released yet. Without this, APNG attachments from
remote posts won’t display as images in frontends.
Fixes: akkoma#657
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.