Consider reserializing type attributes, not just parsing them.

[wisniewskit]

While working on Gecko's Fetch compliance, I noted that there are web platform tests which fail because they expect blob types to be passed through the MIME Sniffing standard's serialization. @annevk suggested that there may have been discussions on doing this reserialization, so I'm filing a bug to verify that option.

[inexorabletash]

Additional context in #43

[annevk]

Aside from Blob, this also affects slice() and File.

I think what should happen is that we store a MIME type internally (I think @mkruisselbrink was planning on adding internal slots) and the type getter serializes it using https://mimesniff.spec.whatwg.org/#serialize-a-mime-type.

I looked into writing a patch for this briefly, but it would end up being quite the rewrite as these bits of the specification are quite dated.

[annevk]

Note that there's tests for this already at /mimesniff/mime-types/parsing.any.js so this is mainly about updating the specification as also discussed at #43 (comment).

[annevk]

Let's duplicate this into that issue as there's no real need to track it separately.