上传空文件失败，$file->isValid()返回0

 $file = $request->file('file');
        if ($file && $file->isValid()) {
            $file->move(public_path().'/files/myfile.'.$file->getUploadExtension());
            return json(['code' => 0, 'msg' => 'upload success']);
        }
        return json(['code' => 1, 'msg' => 'file not found']);