Hi,

actually the only way to get the file listed in a post/page/other is that it has been uploaded there (and i’m not planning to change this logic since it would be confusing).

However, you can change the “owner” of a file with this handful plugin: http://wordpress.org/plugins/unattach-and-re-attach-attachments/

Then, if you re-attach the media file in the post/page you want the file to be listed, you can reach your purpose 🙂

Regards