You have to make sure you click "Upload file" and let it actually upload before you click "Send". There is no check to verify whether there is a file in the upload box prior to sending the post, so sending before uploading results in the file not being attached to your post.
kinda what super Mario fx would've sounded like.
(for those who don't know, smfx was the canceled snes Mario game from which sm64 derived the 3d elements.)
