Bug#995267: devhelp doesn't load the selected content

[Rinat Ibragimov]

Looks like it's sufficient to remove ~/.local/share/mime/application/x-extension-html.xml
and then to regenerate database:

    update-mime-database ~/.local/share/mime

More information is available in bug #722279, which is very similar to this one. In short,
libwebkit2gtk doesn't like "application/x-extension-html" and decides to not render such
documents.