Cannot add custom media types - editor will not link, or allow me to upload media

All: I have followed the directions within the grav documentation for adding a custom media type by adding my mime type to the media.yaml file in the user/config directory. Even though I have done this I still can't link to epub files in the same directory as the page I am trying to use. I also can't upload using the for pay editor. It always says 'invalid media type'. What is the proper procedure for doing this?