使用imagine做图片的缩略图为什么会报出上传的文件不存在 [ 求助交流 ]

public function actionCreate() {
    $model = new Mien();
    if ($model->load(Yii::$app->request->post())) {
        $model->imageFile = UploadedFile::getInstance($model, 'imageFile');
        Image::thumbnail(''.$model->imageFile, 100, 50,ManipulatorInterface::THUMBNAIL_OUTBOUND)
            ->save('thumb_'.$model->imageFile.'.jpg');
        $model->save();
        return $this->redirect(['index']);
    } else {
        return $this->render('create', [
            'model' => $model,
        ]);
    }
}