File Field Type upload_dir

Hi,

Yes, it is possible. You can add the post ID to the custom upload dir when registering the file field. For example:

add_filter( 'rwmb_meta_boxes', 'your_prefix_function_name' );
function your_prefix_function_name( $meta_boxes ) {
    $post_id = null;
    if ( isset( $_GET['post'] ) ) {
        $post_id = intval( $_GET['post'] );
    } elseif ( isset( $_POST['post_ID'] ) ) {
        $post_id = intval( $_POST['post_ID'] );
    }

    $meta_boxes[] = [
        'title'  => __( 'Post Meta', 'your-text-domain' ),
        'id'     => 'post-meta',
        'fields' => [
            [
                'name'                     => __( 'File', 'your-text-domain' ),
                'id'                       => 'file',
                'type'                     => 'file',
                'upload_dir'               => WP_CONTENT_DIR . '/' . $post_id . '/invoices/',
            ],
        ],
    ];
    return $meta_boxes;
}