Hi,
To avoid the complexity, I avoided using predefined variables for things like site number in multisite network. But you can always do that with a little code, like this:
function prefix_get_site_upload_dir() {
return WP_CONTENT_DIR . '/uploads/sites/' . get_current_blog_id() . '/my_uploads/';
}
And in the field settings, set:
'upload_dir' => prefix_get_site_upload_dir(),