Well, at the moment, yes because the quoted sentence in the spec. is not a MUST.
But in AS3 we could make “Use the most specific type” a MUST …
(or whatever it would be in correct english)
And then not all other 1000 implementors are hindered to write a generic validator without the need for ugly code like I just wrote
if (APo.type.indexOf('Image') < 0 && (!APo.mediaType || APo.mediaType.toLowerCase().indexOf('image') !== 0)) {
return ''
}
In above APo.type is always an Array and a generic validator would just look if type is correct for the widget.
However, please note that the mastodon/pleroma examples are
invalid
The specification for mediaType clearly says 
:
When used on an Object, identifies the MIME media type of the value of the content property.
Unfortunately these poor examples try to describe the url property.
And as said, this does not make sense.
And the protocol authors are the clever people …