Contrary to the return type declaration, MimeAnalyzer::improveTypeFromExtension( $mime, $ext ) can return null. This occurs whenever $mime is 'unknown/unknown' and $ext is not a known file extension.
This is surprising behavior. If this method is called with an unknown file extension and a MIME type other than unknown/unknown, it returns the original MIME type unmodified. So "improve" sometimes mean "improve or return unmodified", and other times it means "improve or return null".
The only caller I could find is MWFileProps::getPropsFromPath(), which doesn't appear to be expecting null values.