vanilla-attach-admin-infoa long time ago
Who is able to download attachments is generally configured via the space operation mode - i.e. everyone allowed to view a snip is also allowed to view attachment listings and subsequently download attachments.
PLEASE NOTICE: attachments are stored in a publicly accessible HTTP directory. downloads are not managed by vanilla. the access restrictions imposed by vanilla are therefore security by obscurity at best. so imagine that a long, boring "no warranties" disclaimer follows this paragraph.