[Validator] Add a media_type validator

We could create a new validator called `media_type` whose purpose would be to validate the media type (ie the first part of the content_type, `application`, `image`, `video` and other).
It could make things easier to write, rather than a `content_type: /\Aimage\/.*\z/`.

FYI
```ruby
@mediatype, @subtype = content_type.split('/', 2)
```