Upload only one file

My client only wants users to be allowed to upload one file. How do I turn off the Add new file option?

This is my code:

add_filter( 'rwmb_meta_boxes', 'cnotes_register_meta_boxes' );
function cnotes_register_meta_boxes( $meta_boxes ) {
    $prefix = 'cn_';
    
    $meta_boxes[] = array(
        'title'      => __( 'Competitor Notes', 'textdomain' ),
        'post_types' => 'event',
        'fields'     => array(
            array(
                'name' => __( 'URL', 'textdomain' ),
                'id'   => $prefix . 'url',
                'type' => 'file',
            ),
        )
    );
   
    return $meta_boxes;
}