core: Fix ISO file support to be case insensitive

This patch fixes a bug where an ISO file with an upper-case
file extension was not accepted as a valid ISO.

Change-Id: Iec8df04bacb81db6c872c7920610abdbe1471da4
Bug-Url: https://bugzilla.redhat.com/1154391
Signed-off-by: Eldan Shachar <eshachar@redhat.com>