Zoom out for 16px or smaller images is usually blocked. But allow it for
zoom menu as we allow 50% via zoom menu in other situations where it is
blocked otherwise. Also, this is probably much less complex
implementation wise.
Closes#544
Only use a reliable mime-type. This requires that we load images earlier
in the pipe.
This also fixes warnings about mime types not being available on network
drives like SMB.
Fixes#513
Issue #534
