Fix opening of ISO files

Opening an ISO file was not possible anymore, due to a regression between
shared-mime-info 1.2 and 1.3 which makes application/x-cd-image not detectable
by content. This workaround fix the issue, until we are not sure that
all major distribution ship a fixed shared-mime-info.

BUG: 354344
FIXED-IN: 15.08.3

Differential Review: D453