diff --git a/files/lib/system/attachment/MessageAttachmentObjectType.class.php b/files/lib/system/attachment/MessageAttachmentObjectType.class.php
index dcc2ec8..bb3da60 100644
--- a/files/lib/system/attachment/MessageAttachmentObjectType.class.php
+++ b/files/lib/system/attachment/MessageAttachmentObjectType.class.php
@@ -25,7 +25,7 @@ use wcf\util\ArrayUtil;
 /**
  * Attachment object type implementation for messages.
  */
-class MessageAttachmentObjectType extends AbstractAttachmentObjectType
+final class MessageAttachmentObjectType extends AbstractAttachmentObjectType
 {
     /**
      * @inheritDoc