I changed the subject to “How to make uploads available only to logged-in users” – That’s what you’re trying to do, right?
From discourse-auth-proxy:
That’s not going to do anything like what you describe. It will allow you to protect another site using Discourse as the authentication point.
If what you want to do is make uploads visible only to people who are logged in, you can see Secure Media Uploads.
But, I don’t know of a way that you can have a post be visible but have uploads in it not be available to those who can see the post.
EDIT: But now I remember one!