test(wpt): add mimesniff tests

[marcosc90]

This PR enables 1027/1911 mimesniff tests.

https://github.com/web-platform-tests/wpt/blob/master/mimesniff/mime-types/parsing.any.js

[lucacasonato]

Have you managed to narrow down what the cause of the remaining failures is?

[marcosc90]

Have you managed to narrow down what the cause of the remaining failures is?

It looks like most of the errors are due to missing escaping of special characters. Chrome & Safari have ~700/1911, we're doing pretty good in comparison. I'll start working to get it to 1911 or close to it, in a follow up.

[marcosc90]

After further investigation, it seems that there's a bug in WPT, it expects a TypeError to be thrown for the following value:

\n/x

assert_throws_js(TypeError, () => new Request("about:blank", { headers: [["Content-Type", val.input]] }));

While \n is not a valid value for a header, the specification states that the header value must be normalized, so AFAIK we're doing the right thing by not throwing in that scenario.

To normalize a byte sequence potentialValue, remove any leading and trailing HTTP whitespace bytes from potentialValue.

[marcosc90]

Submitted a PR web-platform-tests/wpt#36359

Update: bug confirmed

[marcosc90]

Have you managed to narrow down what the cause of the remaining failures is?

@lucacasonato The remaining failures are caused by FileAPI spec being broken and incompatible with MIME sniffing spec
See: w3c/FileAPI#43

I'm working in #16233 towards following MIME sniffing spec for Blob.type instead of FileAPI spec in case we want to follow that path, which I think is the best path forward (We can discuss it in that PR)