function add_lightbox_rel($content) {
    global $post;
    $pattern = "/<a(.*?)href=('|\")(.*?).(bmp|gif|jpeg|jpg|png)('|\")(.*?)>/i";
    $replacement = '<a$1href=$2$3.$4$5 rel="lightbox" $6>';
    $content = preg_replace($pattern, $replacement, $content);
    return $content;
}
add_filter("the_content", "add_lightbox_rel");
add_filter("acf_the_content", "add_lightbox_rel");

function add_gallery_rel($attachment_link) {
    global $post;
    $attachment_link = str_replace("<a", "<a rel=\"lightbox\"", $attachment_link);
    return $attachment_link;
}
add_filter("wp_get_attachment_link", "add_gallery_rel");