Fix Bug 938885, use navigator.mimeTypes and new JSON list for plugincheck

I pinged you on IRC but thought I would send you an email with an overview of this PR.

So, for me, I would really appreciate a code level overview of the above indicating whether the JS looks good or, whether there are any things that is just wrong or, can do with a change to improve perf or avoid those middle of the night, wtf? bugs

If however you want to give the code a test run, I would suggest having a look at the README here:
https://github.com/ossreleasefeed/Perfidies-of-the-Web/blob/new-plugincheck/README.md

And please, pester me with any questions you may have. Thanks a lot!