Attach image in media library to custom post type

I have a custom post type called ‘creatures’ that I have enabled post-thumbnails on. It works fine when I attach an image while editing the creature, but when I am in the media library I can’t use the ‘attach’ link with anything other than post or pages.

Is there something I can put in functions.php to have ‘creature’ show up in the attach box that pops up in the media library?

Any help would be appreciated I can’t find anything mentioning it while searching here.