When not logged in, if someone posts an attachment to a post, you see the message "You do not have required permissions to ..." etc.

In order to see the attachments, you login. Then you are redirected back to the forum. Then you go back into the thread and find the post again. THEN you can see the attachment.

In a much better fashion, to post when you are not logged in, by clicking "Post" you are redirected to a login page that redirects you to post.

My suggestion: when you are not logged, the attachment message should look like "You do not have the required permissions to... link*Login to view these attachments/link*"

That way, it's one easy click if you do not have the permissions.