G'Day Mike,
I've noticed on the last few posts I've made with attachments that when I upload the file, close the file upload window and then resume typing in the post window that the file upload window will open again as soon as I type a space. Seems pretty odd!
It's simple to work around... just leave the file upload window open in the background, or try to type the whole post before uploading the attachment (I find I suffer from premature send button compulsive disorder when I do this though!)
Al.