I also just spotted this a few days ago, and worked on this issue. Its corrected for 7.6.1.
There is a bug in 7.6.0 that allows attachments to be added to replies in Gallery Topics.
are intended to all be related to the main Gallery Post.
These are intended to be like a physical gallery. One where you go to this destination and see the works that an artist has created. You can go in and enjoy the works, but you cannot bring your own items and set them up next to the OP's work -- you need to make your own Gallery Topic to display your own gallery.
ex: http://www.deviantart.com/Topic Forums
are where users can contribute additional media to the OP's topic.
ex: your traditional discussion forum.
I know the distinctions between the two are a bit blurred at the moment. Its something that is being worked on as all the newly requested (by other forum admins) attachment features/tools are being added.