Should be MUCH easier

I was expecting to just be able to list new file extensions (like you can in the settings of WordPress Multisite installs) but this plugin, whilst it works, makes something really simple actually really hard.

It is actually far simpler to just add this code to your theme’s function.php file

//* Add more mime types (in this case epub and mobi, but just add |file_extension|and_another to add more)
function addUploadMimes($mimes) {
    $mimes = array_merge($mimes, array(
        'epub|mobi' => 'application/octet-stream'
    ));
    return $mimes;
}
add_filter('upload_mimes', 'addUploadMimes');