This function downloads an attachment or avatar, and increments the downloads.

• Requires the view_attachments permission. (not for avatars!)
• Disables the session parser, and clears any previous output.
• Depends on the attachmentUploadDir setting being correct.
• Is accessed via the query string ?action=dlattach.
• Views to attachments and avatars do not increase hits and are not logged in the "Who's Online" log.