Detect file type before read as stream

[asamuzaK]

Fixes #1646

Detect file type from file extension.

The following extensions will be treated as ASCII file, other as BINARY.
[css, htm, html, js, json, mml, svg, txt, xml, xhtml]

I'm not sure that's enough or too much for linter.
Please tell me.

• [✓] This PR relates to an existing open issue and there are no existing
  PRs open for the same issue.
• [✓] Add Fixes #ISSUENUM at the top of your PR.
• [✓] Add a description of the the changes introduced in this PR.
• [✓] The change has been successfully run locally.
• [✓] Add tests to cover the changes added in this PR.

[asamuzaK]

Sorry, commit by mistake.
It's not related to this PR, reverted.

[rpl]

Hey @asamuzaK Thank you so much for opening this PR and mention it in #1714, unfortunately I did not notice that you were already working on it when I was investigating the issue in web-ext (mozilla/web-ext#1164) and so I worked on it on a different PR.

I ended up to land my PR, mostly because I preferred a more "unit test" testing strategy (which in this PR is more like an integration test, which we should definitely add to this repo, but I would prefer to add them into a separate integration test suite, instead of inside the current unit test suite) and also because of the isAscii check inside of the getFileAsStream (which could be hard to keep in sync over the time), nevertheless I kept into account your proposals from this PR and I tweaked the #1714 accordingly before landing it.

And so, long story short, even if I'm closing this PR, I would say that there is still a bit of your PR in the final #1714 version!

Thanks again!