sul .htaccess della directory di WP:

AddType application/epub+zip .epub
AddType application/x-mobipocket-ebook .mobi

mentre sul file function.php del tema:

function addUploadMimes($mimes) {
    $mimes = array_merge($mimes, array(
        'epub|mobi' => 'application/octet-stream'
    ));
    return $mimes;
}
add_filter('upload_mimes', 'addUploadMimes');