The problem is when you upload attachments one by one it can somehow freeze. That means that you still can upload them but they won't show up in the attachments list (but in fact they are already uploaded). The upload overlay also won't close by itself, you have to click 'Cancel'. The only thing you are left to do is to create a thread (or to post a message) in order to see them.
I am using default uploader, however I have Flash installed with FlashControl but XenForo still won't allow me to use flash uploader. Browser: Google Chrome 28 on Win 8.