Add a way to reset the singleton

src/Symfony/Component/HttpFoundation/File/MimeType/MimeTypeGuesser.php

@@ -67,6 +67,13 @@ class MimeTypeGuesser implements MimeTypeGuesserInterface
         return self::$instance;
     }
 
+    /**
+     * Resets the singleton instance.
+     */
+    public static function reset() {
+        self::$instance = null;
+    }
+
     /**
      * Registers all natively provided mime type guessers.
      */

src/Symfony/Component/HttpFoundation/Tests/File/FileTest.php

@@ -45,6 +45,18 @@ class FileTest extends \PHPUnit_Framework_TestCase
         $this->assertEquals('gif', $file->guessExtension());
     }
 
+    public function testGuessExtensionWithReset() {
+        $file = new File(__DIR__.'/Fixtures/other-file.example');
+        $guesser = $this->createMockGuesser($file->getPathname(), 'image/gif');
+        MimeTypeGuesser::getInstance()->register($guesser);
+
+        $this->assertEquals('gif', $file->guessExtension());
+
+        MimeTypeGuesser::reset();
+
+        $this->assertNull($file->guessExtension());
+    }
+
     public function testConstructWhenFileNotExists()
     {
         $this->setExpectedException('Symfony\Component\HttpFoundation\File\Exception\FileNotFoundException');