Media attributes missing

[bocops]

Follow-up from #660, the Mastodon documentation example JSON for audio attachments lists several attributes that we're currently not making available, for example:

• audio_encode
• audio_bitrate
• audio_channels

There's also at least fps in the GIFV example above that we're currently not handling well - and it is unclear to me if these examples list all possible attributes or if there might be others. From the documentation and my limited understanding of Ruby code, it seems as if Mastodon is just making available whatever Paperclip returns for any given media attachment.