bug symfony#15110 Add a way to reset the singleton (dawehner)

This PR was submitted for the 2.8 branch but it was merged into the 2.3 branch instead (closes symfony#15110).

Discussion
----------

Add a way to reset the singleton

## Problem
The mime type guesser is a singleton, which as we know, causes all kind of problems.
One problem Drupal ran into is the problem that for tests we continuesly registered more and more mimetype guessers,
once everytime we rebuild the container as part of your testing.

Once that we had that, with enough test functions + made one of the mimetype guessers as proxy, we ran into a memory leak
of storing the container as part of the mime type singleton.

See https://www.drupal.org/node/2408371

## Solution
Allow to reset the singleton.

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets |
| License       | MIT

Commits
-------

0096266 Add a way to reset the singleton

Author: fabpot

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

@@ -67,6 +67,13 @@ public static function getInstance()
         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 @@ public function testGuessExtensionIsBasedOnMimeType()
         $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');