Implement MIME sniffing

[Ms2ger]

There's https://mimesniff.spec.whatwg.org/ , but it's not quite clear to me where it's invoked.

[jdm]

N.B. we should revert the changes to mime_sniffing_font_context.html from #13931 as part of this work.

[dpyro]

I’m taking a look at it right now and seeing what can be done to simplify and solidify the implementation.

[KiChjang]

Does components/net/mime_classifier.rs not contain an implementation of the MIME sniffing algorithm already?

[dpyro]

Looks like it, @KiChjang. Personally I’d like it decomposed down into smaller submodules. Then again that may just be bikeshedding on my part. Besides, “if it ain’t broke, don’t fix it”.

[KiChjang]

I'm actually more inclined to make it conform more to the algorithms as defined by the spec, but I'm not sure how much effort is required to do so.

[jdm]

It's supposed to already conform. This issue is specifically talking about integrating the MIME sniffing into the fetch network stack, now that the old network stack that supported it has been removed.

[dpyro]

This is blocking #14521 if we want to do it properly, since nosniff needs to compare the Content-Type header to the result of mimesniff.

[Ms2ger]

If integrating this into the fetch stack at all is even the right answer; it might be better to integrate it into the code that calls into the fetch stack instead. After all, the fetch stack doesn't interpret the bytes it returns.