$args = array(
            'labels' => $labels,
            'public' => true,
            'publicly_queryable' => true,
            'show_ui' => true,
            'query_var' => true,
            'rewrite' => array('slug' => 'slider'),
            'capability_type' => 'post',
            'hierarchical' => false,
            'menu_icon' => 'dashicons-images-alt',
            'has_archive' => true,
            'supports' => array( 'title', 'excerpt', 'thumbnail'),
            'taxonomies' => array()
        );

    register_post_type( 'a21_slider' , $args );

....
    $prefix = '_a21_slider_';

    $slides_box = new_cmb2_box( array(
        'id' => $prefix . 'slides',
        'title' => __( 'Slides', 'a21-slider' ),
        'object_types' => array( 'a21_slider' ),
        'context' => 'normal',
        'priority' => 'default',
        'show_names' => true
      ));

    $slides_group = $slides_box->add_field( array(
        'id'          => $prefix . 'slides',
        'type'        => 'group',
        'description' => __( 'Select your slides', 'a21-slider' ),
        'options'     => array(
            'group_title'   => __( 'Slide {#}', 'a21-slider' ),
            'add_button'    => __( 'Add Another Slide', 'a21-slider' ),
            'remove_button' => __( 'Remove Slide', 'cmb2' ),
            'sortable'      => true,
        ),
    ) );


    $slides_box->add_group_field( $slides_group, array(
            'name' => __( 'Slide', 'a21-slider' ),
            'id'   => $prefix . 'slide',
            'type' => 'file'
        ) );
}