Think I just figured it out (for my system at least). I'm running Firefox under Windows 7 on a big 8-core AMD desktop machine.
By default, I use 'private' windows in Firefox for everything... in fact, that's the main reason I use Firefox, because of their emphasis on privacy and anti-tracking. As an experiment, I just tried opening the Den in a normal (i.e., 'non-private') window. The attachment box now shows up.
Woodgrain, you might try the same thing if you're running Firefox. You would only need to do it when you need to add some pictures...
JB