If one sets the attachment permissions for users of a space to allow removal, then as far as I can tell, any user can delete and attachment and it is completely gone. So the only way to recover the lost attachment is by restoring from a backup. On the other hand, if one does not allow attachment removal, then a Creator of an attachment cannot remove that attachment, so it is permanently up unless the administrator removes it. It would be convenient if there was an intermediate level, for which only the Creator and Space Administrator can permanently remove attachments.