We should be able to pass the mimesniff/mime-types/charset-parameter.window.html WPTs if we use our new MIMEType parser here:

• https://searchfox.org/mozilla-central/source/netwerk/protocol/http/nsHttpResponseHead.cpp#625

We could also pass a lot of the other MimeSniff tests by using it for blob content types, but unfortunately those tests conflict with the File spec (because the File spec expects blobs to only be lowercased and rejected if they have non-ASCII characters, while the MimeSniffing spec seems to expect them to be normalized, as if parsed and then serialized).

https://searchfox.org/mozilla-central/rev/8b470bef89879d6c9248e183562f77fd8da9b926/netwerk/protocol/http/nsHttpResponseHead.cpp#634

net_ParseContentType(val, mContentType, mContentCharset, &dummy);

We need to do something like this:
https://searchfox.org/mozilla-central/rev/503938c13ef2dd174705dc0f6d0683ae43074ccc/netwerk/protocol/data/nsDataHandler.cpp#187-194

if (mozilla::UniquePtr<CMimeType> parsed = CMimeType::Parse(mimeType)) {
  parsed->GetFullType(aContentType);
  if (aContentCharset) {
    parsed->GetParameterValue(kCharset, *aContentCharset);
  }
  if (aMimeType) {
    parsed->Serialize(*aMimeType);
  }

Note that I actually have a patch in the works here already, but there were still try-failures I haven't accounted for: https://treeherder.mozilla.org/jobs?repo=try&revision=356a369dc72bbecb6f0a525ad7c2dca464a33f27