diff --git a/src/Symfony/Component/Mime/MimeTypes.php b/src/Symfony/Component/Mime/MimeTypes.php
index dfdea060e466d..33d51d7f5b5f3 100644
--- a/src/Symfony/Component/Mime/MimeTypes.php
+++ b/src/Symfony/Component/Mime/MimeTypes.php
@@ -51,7 +51,8 @@ public function __construct(array $map = [])
             $this->extensions[$mimeType] = $extensions;
 
             foreach ($extensions as $extension) {
-                $this->mimeTypes[$extension] = $mimeType;
+                $this->mimeTypes[$extension] = $this->mimeTypes[$extension] ?? [];
+                $this->mimeTypes[$extension][] = $mimeType;
             }
         }
         $this->registerGuesser(new FileBinaryMimeTypeGuesser());
diff --git a/src/Symfony/Component/Mime/Tests/MimeTypesTest.php b/src/Symfony/Component/Mime/Tests/MimeTypesTest.php
index a736dbebbae0f..6598fed85e089 100644
--- a/src/Symfony/Component/Mime/Tests/MimeTypesTest.php
+++ b/src/Symfony/Component/Mime/Tests/MimeTypesTest.php
@@ -62,4 +62,15 @@ public function testGetMimeTypes()
         $this->assertContains('image/svg', $mt->getMimeTypes('svg'));
         $this->assertSame([], $mt->getMimeTypes('symfony'));
     }
+
+    public function testCustomMimeTypes()
+    {
+        $mt = new MimeTypes([
+            'text/bar' => 'foo',
+            'text/baz' => ['foo', 'moof'],
+        ]);
+        $this->assertContains('text/bar', $mt->getMimeTypes('foo'));
+        $this->assertContains('text/baz', $mt->getMimeTypes('foo'));
+        $this->assertSame(['foo', 'moof'], $mt->getExtensions('text/baz'));
+    }
 }