Mention IANA Media Types vocabulary in `fileMediatype`

[peterdesmet]

We should probably mention to use the IANA Media Types vocabulary for values in the fileMediatype term. In addition, we can add a constraint to validate values. Providing an exhaustive list in an enum might not be scalable, but a regex pattern can go a long way.

Suggestion:

Mediatype of the media file. Expressed as an IANA Media Type.

Regex

(image|video|audio)\/.*

Will match values such as:

video/1d-interleaved-parityfec
video/BT656
image/g3fax
audio/ATRAC-ADVANCED-LOSSLESSv