How do I get the real image url on forum?

Is there a way to get the real image URL of an image attachment in the forums module? The URL EE provides causes a problem in Fancybox:

http://www.biertje-sateetje.nl/forums/viewthread/7/ (click little dutch flag, big image may crash your browser)

Using the normal URL works fine, but I don’t know how to retrieve this. In the given URL I do have the filename, but no file extention (jpg, png, etc)