Reviewed by Simon Hausmann.

        As Qtish implementation of MIMETypeRegistry::getMIMETypeForExtension()
        returns the application/octet-stream mimetype when it can't associate
        extension with any mimetype, it can happen that the application/octet-stream
        mimetype will hit the list of supported image formats. For instance,
        it is possible when QImageReader or QImageWriter support an extension
        that is not in the extensions map.

        Make sure that this mimetype is not treated as displayable image type.

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@43214 268f45cc-cd09-0410-ab3c-d52691b4dbfc