disc: Don't parse anything but ISOs as ISOs
disc: Don't parse anything but ISOs as ISOs

If we encountered a non-ISO9660 image masquerading as one,
but in a format supported by libarchive, we would have ended
up tagging it as a DVD without being able to read it later on.

https://bugzilla.gnome.org/show_bug.cgi?id=702828

MathieuDuponchelle/totem-pl-parser,MathieuDuponchelle/totem-pl-parser,MathieuDuponchelle/totem-pl-parser,MathieuDuponchelle/totem-pl-parser,pwithnall/totem-pl-parser,pwithnall/totem-pl-parser