The code supplied for plupload_default_params was a little off. get_current_screen() returns an object.
add_filter('plupload_default_params', function($params) {
if (!function_exists('get_current_screen')) {
return $params;
}
$current_screen = get_current_screen();
if ($current_screen->id == 'user-edit') {
$params['user_id'] = $_GET['user_id'];
} elseif ($current_screen->id = 'profile') {
$params['user_id'] = get_current_user_id();
}
return $params;
});
With this $_POST[‘user_id’] is set correctly and is available during wp_handle_upload_prefilter
