The problem is that XenForo receives a valid flash response, so it displays the Flash uploader and provides a working script to the computer. If the computer's flash mishandles or fails to properly parse or display the upload button, XenForo doesn't receive any indication that it failed to enact a fall back. These types of issues are always browser specific (it handles something one way and no other way can work, etc.) or on your computer and not on the majority of users using the product.