Yii2。为什么需要上传两次才能生效？

// 前端表单
<?= $form->field($model, 'file')->fileInput() ?>

// 后端控制器
public function actionUpload()
{
    $model = new UploadForm();

    if (Yii::$app->request->isPost) {
        $model->file = UploadedFile::getInstance($model, 'file');
        if ($model->file && $model->validate()) {
            $model->file->saveAs('uploads/' . $model->file->baseName . '.' . $model->file->extension);
            return $this->redirect(['index']);
        }
    }

    return $this->render('upload', ['model' => $model]);
}

if ($model->file && $model->validate()) {
    $filePath = 'uploads/' . $model->file->baseName . '.' . $model->file->extension;
    if ($model->file->saveAs($filePath)) {
        Yii::info("File uploaded successfully: " . $filePath);
    } else {
        Yii::error("Failed to save file: " . $filePath);
    }
}

$('#upload-form').on('submit', function(e) {
    e.preventDefault();
    var formData = new FormData(this);

    $.ajax({
        url: '/upload',
        type: 'POST',
        data: formData,
        processData: false,
        contentType: false,
        success: function(response) {
            alert('File uploaded successfully!');
        },
        error: function(xhr, status, error) {
            alert('An error occurred while uploading the file.');
        }
    });
});