Hardcode acceptable attachment extensions to common images

files/lib/system/attachment/MessageAttachmentObjectType.class.php

@@ -79,6 +79,17 @@ class MessageAttachmentObjectType extends \wcf\system\attachment\AbstractAttachm
 		return 1;
 	}
 
+	/**
+	 * @inheritDoc
+	 */
+	public function getAllowedExtensions() {
+		return [ 'png'
+		       , 'gif'
+		       , 'jpg'
+		       , 'jpeg'
+		       ];
+	}
+
 	/**
 	 * @inheritDoc
 	 */