I observed for a long time a black square next to one thread. Now that I actually bothered, it’s the little picture that marks when there is an attachment in the thread. In firefox, it seems too small (hence the black “clump”) in Internetexplorer it’s okay, however, still not distinguishable at all.
See picture:
I think the size might be just a problem with supplying the width and height, since it’s probably not needed for the faulty ie code.