I had made an attachment in an admin area of our forum, it was a picture of a business card design. When I quoted that post and pasted it in the public area, only people with permission to view the admin thread could see that attachment - everyone else saw a broken link.
Also I made a template mod so that existing attachments show up when they are quoted or re-used, but I verified that this also happens without that mod (one of my sites has it, one does not).
I made an attachment in an admin thread, then quoted that and made a new thread in public and inserted the quote.
On another browser not logged in, permissions set for guest to view content and attachments, clicking the link takes you to a "you must be logged in" page / error.
If I move the thread from the admin area to the public area, attachments are visible, no problem.
So if I had to guess, there are permissions tied to attachments, depending on where they are made?