bug #29975 [Mime] fix adding extensions when creating a MimeTypes ins…

…tance (nicolas-grekas)

This PR was merged into the 4.3-dev branch.

Discussion
----------

[Mime] fix adding extensions when creating a MimeTypes instance

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

Commits
-------

6c7d034 [Mime] fix adding extensions when creating a MimeTypes instance

src/Symfony/Component/Mime/MimeTypes.php

@@ -34,13 +34,14 @@
  */
 final class MimeTypes implements MimeTypesInterface
 {
+    private $extensions = [];
     private $guessers = [];
     private static $default;
 
     public function __construct(array $map = [])
     {
         foreach ($map as $mimeType => $extensions) {
-            self::$extensions[$mimeType] = $extensions;
+            $this->extensions[$mimeType] = $extensions;
         }
         $this->registerGuesser(new FileBinaryMimeTypeGuesser());
         $this->registerGuesser(new FileinfoMimeTypeGuesser());
@@ -71,7 +72,7 @@ public function registerGuesser(MimeTypeGuesserInterface $guesser)
      */
     public function getExtensions(string $mimeType): array
     {
-        return self::$map[$mimeType] ?? [];
+        return $this->extensions[$mimeType] ?? self::$map[$mimeType] ?? [];
     }
 
     /**