You should already be able to. If you're starting a new thread or have clicked the "Post Reply" icon (note, this doesn't work in quick reply unless you click the "Go Advanced" button"), type in the text you want in the message box, then scroll down to the "Manage Attachments" button. Click on it and a window should pop up (make sure you allow popups from the tfp if you've done all of the above so far but aren't getting the popup). Then, browse to the file you want to upload and upload it. (make sure it's 100 kB or under). That should be it.
