[ticket/12071] Skip tests that depend on fileinfo and fix expected results

Tests that depend on fileinfo being enabled will now be skipped to prevent
fatal errors while running the test suite. The expected results will be
modified to "application/octet-stream" in test_guess_files() if only the
extension_guesser is available due to the non-existing extension of those
files.

Ticket: http://tracker.phpbb.com/browse/PHPBB3-12071

PHPBB3-12071