What happens when "mediaType" changes #1267

We need to identify what the process is for a change to "mediaType". Currently - the field is updated but "DeliveryChannels" are never updated. e.g. if I submit "mediaType": "image/png", default behaviour would be to give this asset "iiif-img" and "thumbs" delivery-channels. If I make a subsequent request to update this to "audio/mp3" the DB record is updated and it's sent to the Engine, however Engine will fail as it's not an image.

Do we want to reset the default DeliveryChannels if mediaType changes? Possibly only if they're not the default (how would this work in the above example if we'd specified "file" delivery channel initially, this isn't the default but was maybe the customers intention to keep? Prevent "mediaType" from changing, other than subtype (ie image/png => image/jpeg is valid but application/mp4 => audio/mp4 is not).