The File attachment "Add/Update" button is not enabled on those forums. It works on other forums, such as "Logos 4 Windows":

It appears that it has to be enabled on a case-by-case basis for each forum. What a pain. Why not have it be on by default for all forums?
Yes, you can use the paperclip icon to do attachments, but there are times when the other feature is a better way to do it.