Better support for audio only files

[heggemsnes]

Is your feature request related to a problem? Please describe.
Mux is great but a lot of time we find ourselves using it for audio only. This currently kinda works but is a bit hacky and would be great if it is better supported.

Describe the solution you'd like
Allow MP3s (and perhaps other Mux supported files). Be able to select MP3 files and give a bit of love to the preview of these files.

Describe alternatives you've considered
Currently the plugin kinda supports MP3s as Mux does it already. It is obviously not really meant to work. MP3s are disabled through the file selector dialog, but works when dragging a file it works fine (please don't patch this without adding better support):

Unfortunately the positioning here is not ideal as the actions is quite hidden:

Otherwise this works fine and can be used with the Mux player with audio=true prop.

Additional context
Great work with the plugin! (hope to see native thumbnail time support in the future as we need some custom input components now)

[narkowicz]

We're also starting to use mux for audio upload via Sanity and hit this problem. Updating to 2.8.0 improves on the critical issue of the concealed options drop down:

Although the player now hangs off the bottom of the Sanity field, reducing margin between the following field. The player also hangs below the thumbnail in the plugin's media browser:

Further, the player also displays a broken poster frame, perhaps a regression on #128 (comment) ?

Would be amazing to see better audio support in this plugin—in our case we're adding audio descriptions for accessibility reasons and the ability to restrict to audio only uploads (i.e. type: 'mux.audio') would create a much better content editor workflow.