Add custom mime types to WordPress

//The following goes in a themes functions file or a custom hooks plugin

function custom_upload_mimes ( $existing_mimes ) {
    $existing_mimes['epub'] = 'application/epub+zip';
    $existing_mimes['mobi'] = 'application/x-mobipocket-ebook';

    return $existing_mimes;
}

add_filter('upload_mimes', 'custom_upload_mimes');