In the user control panel (UCP), users can get the list of their attachments, view them, erase them and go to the topics where they inserted these attachments.

What happens if a moderator erases a post/topic with an attachment (without removing the post/topic from the database) or moves that post/topic to a forum not visible by the author (a forum visible only for administrators, moderators or another special users group) ?

The post will be hidden for the author, but he or she can still view the attachment in the UCP! Therefore, he or she can be aware the post hasn't actually been erased. He or she can still delete the attachment (unless the post has been locked). Later, if the related topic is renamed, he or she can know the new topic name. Finally, if the user click to the topic link, he or she gets either "the requested topic does not existed" or "you are not authorised to read this forum".

I think the attachments of hidden posts (removed not "permanently" or removed to a hidden forum) shouldn't be visible in the User Control Panel.