diff --git a/src/Symfony/Component/Mime/MimeTypes.php b/src/Symfony/Component/Mime/MimeTypes.php
index 5dd2aba73ed99..5f057a99ec183 100644
--- a/src/Symfony/Component/Mime/MimeTypes.php
+++ b/src/Symfony/Component/Mime/MimeTypes.php
@@ -48,7 +48,6 @@ final class MimeTypes implements MimeTypesInterface
     public function __construct(array $map = [])
     {
         foreach ($map as $mimeType => $extensions) {
-            $extensions = \is_string($extensions) ? [$extensions] : $extensions;
             $this->extensions[$mimeType] = $extensions;
 
             foreach ($extensions as $extension) {