未定义索引：扩展

Question

遇到PHP错误

public function actionDownload($id)
{
    $model=work::model()->findByPk($id);
    $file = Yii::app()->params['fileHomeAbs'].'work/'.$model->NM_FILE;
    $info = pathinfo($file);
    $ext = '.'.$info['extension'];
    if(file_exists($file)){
        return Yii::app()->getRequest()->sendFile($model->NM_FILE, @file_get_contents($file));  
    }
}

未定义索引：扩展名