don't panic if what we thought was a zip file is actually a rar

[tinruufu]

i've run into instances of people saving rar-compressed files as .cbz, and there's no reason we shouldn't handle this (and vice-versa, and if we add additional format support in future)

the reason we only check extensions is that it's way, way quicker to list huge directories than to read the headers of every single file, but that doesn't matter once we've committed to extracting one particular file; we may as well work out what the thing is manually

[tinruufu]

it depends how we solve this, but perhaps related: we don't handle multipart archives very well if we pick something higher than part one