works for new posts but not new pages or new media

I second this– I also need this plugin to work for custom post types, but it doesn’t right now. I can hack it, but it’d be nicer if the plugin would support this. You could do it by changing lines 105-107 to:

if( !post_type_supports($data['post_type'], 'author') ):
				return $data;
			endif;

nice. might I suggest a pull request? https://github.com/birgire/wp-default-author