Add hook after file upload/update

I want to trigger a function after a file has been updated or uploaded. What I want to do is send a notification to the people who have access to this file, notifying them of its existence.

Assuming that documents are just posts, I thought I could do this:
add_action('wp_transition_document_status', 'send_file_notification');

But my send_file_notification function is not called. Does anyone have an idea how to fix/create this?

http://wordpress.org/extend/plugins/wp-document-revisions/