文件上传验证失败 [ 2.0 版本 ]

加上了$model->validate()验证，文件上传校验失败，提示未上传文件。像是个bug。去掉验证到是能正常上传

控制器

public function actionTest()
{

        $model = new DynamicModel();
        $name='imagefile';
        $model->defineAttribute("$name");
        $model->addRule("$name", 'file',['skipOnEmpty' => false,'extensions' => 'png, jpg']);
        
            if (Yii::$app->request->isPost && $model->validate()) {
            $model->imagefile = UploadedFile::getInstance($model, 'imagefile');
          
          if (Yii::$app->request->isPost && $model->load(Yii::$app->request->post()) && $model->validate()) {
                // 文件上传成功
                //return;
            }
        }
          return $this->render('test', [
                'model' => $model,
            ]);/* */
         //var_dump($model->imagefile);
}

view

<?php
use yii\widgets\ActiveForm;
?>

<?php $form = ActiveForm::begin(['options' => ['enctype' => 'multipart/form-data']]) ?>

<?= $form->field($model, 'imagefile')->fileInput() ?>

<button>Submit</button>

<?php ActiveForm::end() ?>