I see, and I can reproduce it with version 5.1.0. But I can't do anything about it.
Here is a link which describes the problem.
If you can get rid of the suffix attachment like .html, this will work immediately.
If you can't, please contact the Kunena guys. The problem is in plugins\system\kunena\kunena.php:65. The code retrieves an HTML document even if the requested format is different. As a result, you get a full page instead of a small snippet.
If I see the code correctly, you can get the Kunena "powered by"-plugin and it will work since it is no longer necessary for the system plugin to add links to the document.