I wouldn't consider this a bug personally as one generally finishes their post before posting it and that in my opinion would include attachments. I don't want this changed myself as disabling the button until uploads have finished may introduce problems as well as the fact that if you post and for some reason your attachment doesn't show (clicking submit before the upload is finished.), you can always edit it after. If your uploading something and you submit a form that navigates to a new page whatever your doing is halted just like clicking on a link in a webpage before the page finishes loading, just apply that logic to an upload form. To me this is more of a user issue that can easily fixed by the user and doesn't need changing.