Is use wpForo combined with the Advanced Attachments addon.
If you add a new topic from out the main forum section the 'Attach Filles' button is not shown. But if you go into the subforum and then add a topic it is shown. See the attached file.
And the second file (because the addon isn't active on your own forum 😉 ) where you see that it works in the subforums.
Great products by the way 🙂