MetaBox

add_filter( 'rwmb_meta_boxes', 'your_prefix_register_meta_boxes' );

function your_prefix_register_meta_boxes( $meta_boxes ) {
    $prefix = '';

    $meta_boxes[] = array (
        'title' => esc_html__( 'Document Options', 'text-domain' ),
        'id' => 'document-options',
        'post_types' => array(
            0 => 'document',
        ),
        'context' => 'normal',
        'priority' => 'high',
        'validation' => array (
            'rules'  => array(
                'file_upload_x3h8w2hgtqg' => array(
                    'extension' => "pdf",
                    // More rules here
                ),
                // Rules for other fields
            ),
        ),
        'fields' => array(
            array (
                'id' => $prefix . 'wysiwyg_rv3nfg4gl9',
                'name' => esc_html__( 'Description of Document Set', 'text-domain' ),
                'type' => 'wysiwyg',
                'options' => array(
                    'media_buttons' => false,
                ),
            ),
            array (
                'id' => $prefix . 'file_upload_x3h8w2hgtqg',
                'type' => 'file_upload',
                'name' => esc_html__( 'File upload', 'text-domain' ),
                'max_status' => 1,
                'force_delete' => 1,
            ),
            array (
                'id' => $prefix . 'user_c33ln3ltujc',
                'type' => 'user',
                'name' => esc_html__( 'User', 'text-domain' ),
                'field_type' => 'select_advanced',
                'multiple' => true,
            ),
        ),
    );

    return $meta_boxes;
}