I've saw a few with this problem including myself. An attachment is uploaded and submitted...it dissappears yet is in the thread according to the list that shows all our attachments...so what gives?
Got me It happened to me the other day in another post also. You also can't reupload it because it tells you its already in such and such thread. So you would have to rename it and try again. To big of a headache I just gave up.