[Validator] Add a media_type validator

[Mth0158]

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

@mediatype, @subtype = content_type.split('/', 2)