After revision 4f9644ce37ed42380a0c013aac4d9febbb552426, html, js, html, etc files will all be verified with content verification.  This means that any icon set to a [html html js] file will fail verification.

With this being the case, we should at minimum warn developers when they load the extension, and likely make this a flat-out load failure (so that the extension doesn't get added, rather than having it be added and immediately disabled for corruption).

Forking from https://bugs.chromium.org/p/chromium/issues/detail?id=809088#c11.