[whatwg/fetch] Block additional script MIMETypes (audio, video, csv) (#379)

Closes  #337.
I also added audio/video, because per telemetry they are probably unused. I think it makes sense to block them alongside of `image/`. `text/csv` is blocked to mitigate the issue described in [Bug 1048535](https://bugzilla.mozilla.org/show_bug.cgi?id=1048535).

@mikewest Are you on board with this?
You can view, comment on, or merge this pull request online at:

  https://github.com/whatwg/fetch/pull/379

-- Commit Summary --

  * block additional script MIMETypes (audio, video, csv)

-- File Changes --

    M Overview.src.html (4)

-- Patch Links --

https://github.com/whatwg/fetch/pull/379.patch
https://github.com/whatwg/fetch/pull/379.diff

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