public static function download($idUser, $idCourse)
 {
     $app = \Slim\Slim::getInstance();
     if (!Course::find($idCourse)) {
         $app->response->setStatus(404);
         return json_encode("Course not found");
     }
     if (!Enrollment::where('user_id', '=', $idUser)->where('course_id', '=', $idCourse)->count()) {
         $app->response->setStatus(401);
         return json_encode("You have to enroll this course first.");
     }
     FileController::downloadCourse($idCourse);
 }