add_meta_box not working for attachment post type

[marcelo2605]

I'm using this code to create a text field for attachment post type:

$foo = new \Super_Custom_Post_Meta( 'attachment' );
$foo->add_meta_box( array(
        'id' => 'book',
        'context' => 'normal',
        'fields' => array(
            'usuario' => array( 'label' => '', 'type' => 'text')
        )
    ));

The input appear on attachment page, but the values are not saved on db.