Unite Test: Serialization of 'Symfony\Component\HttpFoundation\File\UploadedFile' is not allowed

I'm trying to write white test to test my API with file uploads.

I'm following the docs about this using basic client request, not crawler.

The unit test is:

class RecordsControllerTest extends WebTestCase {

private $client;

public function __construct() {
    parent::__construct();
    $this->client = self::createClient();
    $this->client->insulate();
}

public function testApiPostUpload($params){
    $fileToUpload = realpath(__DIR__.'/../../resources/mpthreetest.mp3');
    $file = new UploadedFile(
        $fileToUpload,
        'mpthreetest.mp3',
        MimeTypeGuesser::getInstance()->guess($fileToUpload),
        filesize($fileToUpload)
    );
    $this->client->request('POST', '/records/'.$params['createdRecordId'].'/upload', array(), array('file' => $file) );

    $this->assertEquals(200, $this->client->getResponse()->getStatusCode());
}
}

When I execute the test I receive an error:

Exception: Serialization of 'Symfony\Component\HttpFoundation\File\UploadedFile' is not allowed

/path/to/project/vendor/symfony/symfony/src/Symfony/Bundle/FrameworkBundle/Client.php:165
/path/to/project/vendor/symfony/symfony/src/Symfony/Component/BrowserKit/Client.php:348
/path/to/project/vendor/symfony/symfony/src/Symfony/Bundle/FrameworkBundle/Client.php:143
/path/to/project/vendor/symfony/symfony/src/Symfony/Component/BrowserKit/Client.php:313
/path/to/project/src/Bundle/Tests/Functional/Controller/RecordsControllerTest.php:182

I have found this question for about the same error, but in this case the request is not sent to the controller and the problem is not the entity and implementing serialization.

Anyone who knows how to fix this?

Anyone who managed to make unit test for uploading file in symfony 2?