I started this idea after I was replying to another Idea here:
Once I started this reply I decided to post this which is what I believe is the starting point to the other idea. I apologize if it is content regurgitation, but I do believe it merits a separate Idea.
First I ask the question, what is the single boiled down difference of attachment and the other post_types? I say it is the relationship to a file. An attachment, and the attachment edit, add new pages all have to do with some sort of file relationship. While other post_types are about creating content and relating attachments to that content.
I think we should be able to declare whether a post_type is content or file related. Which essentially defines whether it behaves like an attachment or like a regular post. With this piece of functionality in place, I believe it would make the brainstorm in adding media tags much more productive. Registering a taxonomy would be able to check automatically whether the post_type it is related to is related to a file (attachment like). Then the taxonomy registration would be built-in to the attachment edit, add new pages just as they are the post edit, add new pages.
Think of the ability to add private media.
Think of the ability of differentiating content media and user submitted media.
Think of the ability to control the media edit, add new pages similarly as custom post types are now for the content related edit, add new pages.