Refactor mime type detection to be reusable; skip test in PHP 5.2

The skipped test is testing behavior only applicable to PHP 5.3.

Even the part of the code which is being tested (https://github.com/zendframework/zf1/blob/master/library/Zend/Validate/File/MimeType.php#L160) is only executed on PHP 5.3, so the test doesn't make sense on 5.2 either.