public function testGuessExtensionIsBasedOnMimeType() {
$file = new File(__DIR__ . '/Fixtures/test');
$guesser = $this
->createMockGuesser($file
->getPathname(), 'image/gif');
MimeTypeGuesser::getInstance()
->register($guesser);
$this
->assertEquals('gif', $file
->guessExtension());
}