How to detect when a file has been uploaded?

The ACF documentation says, the native upload of WordPress is triggered, so you can use wp_handle_upload:

function my_cool_func() {
    // whatever it is that you want to do, do it here
} // function my_cool_func
add_action('wp_handle_upload', 'my_cool_func');

Or maybe you want/have to hook into add_attachment…