It does sound like a problem on your computer, but Google shows that it isn't an uncommon problem on XenForo forums either.
"Just doesn't work" covers a multitude of sins. For instance, clicking on it and nothing happens, vs clicking on it and getting the file selection window but being unable to select, vs selecting and it not displaying. A lot of the complaints in that Google search show files attaching but having a line through their name and not displaying.
My natural suspicion is that your computer has performed some sort of automatic update while you weren't looking, but I am not savvy enough to know what it might be.
There are a lot of much more computer literate folk here, maybe someone else can chime in?
Chris