"As a Reader, I want to know the internet media type of files related to a page, so I can start the right program to read or display the file."

The media links endpoint T230845 has media types like "drawing" or "bitmap" instead of the standard formats like "image/png" or "audio/ogg".

Since those media type tags are so useful and prevalent, we should support them in our output. The current endpoint is marked as /v1, so we can't just replace the mediatype property.

So, we need to add a new property, mime_type, to hold the MIME type for the file.