mime: ignore .js => text/plain in Windows registry

[ferrmin]

This seems to be a common registry misconfiguration on Windows.

Fixes golang#32350

Change-Id: I68c617c42a6e72948e2acdf335ff8e7df569432d
Reviewed-on: https://go-review.googlesource.com/c/go/+/406894
Reviewed-by: Michael Knyszek [email protected]
TryBot-Result: Gopher Robot [email protected]
Run-TryBot: Ian Lance Taylor [email protected]
Reviewed-by: Damien Neil [email protected]

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