Implement new attachment type audio

[connyduck]

In the next release mastodon will add audio attachment support. mastodon/mastodon#11123

Note that it is not yet decided which format the audio will have, see mastodon/mastodon#11131

[koyuawsmbrtn]

Suggestion: Allowing attachments to be audio files and being able to record voice notes to send them around

[connyduck]

-> #1337

[novadeviator]

audio support is now enabled in mastodon. it's mp3 format. this issue is not clear in terms of what is supposed to be implemented - playing or/and uploading audio. #1337 is for uploading, but playback of audio does not work in Tusky currently if i'm not mistaken.

[connyduck]

This issue is about displaying the attachments correctly in timelines & playing audio.