Bug 2137: Improve license check

Rather than relying on a case sensitive extension (.jpg, etc)
this change introduces the use of file mime-type to better
determine if a file requires a license or not.

Change-Id: I24492c584c0d124c47d542cb8f8019ae56642682
Signed-off-by: Mark Beierl <mark.beierl@canonical.com>