The files are displayed correctly, and are available on the server (I can copy them to another machine and view them). However, when clicking on the file, the light box displays the following error:
This image failed to load.
If I attempt to open the image in a new tab/window, I get a "404 Not Found" exception.
I've checked the permissions throughout the Jenkins install and everything seems to be in order (however, this an instance that doesn't use any kind of authentication; everyone is the anonymous user with full permissions).
Clicking the "attachments" link to the left yields even more interesting results:
http://junit/<namespace>/<test class name>/attachments
It appears that the link is rather incomplete, and doesn't reference the actual Jenkins instance I'm using.
If I've missed anything, or you need additional information, please let me know!