creating symfony image fixture for SonataMediaBundle - php

Question

I'm trying to create an image fixture for SonataMediaBundle. The media is saved in the database and shown in the admin interface, but the image is missing. There is no difference between the records created by fixture and admin interface.
Here is my Fixture in AppBundle\DataFixtures\ORM\LoadMediaBundleMediaData
<?php
namespace AppBundle\DataFixtures\ORM;
use Doctrine\Common\DataFixtures\AbstractFixture;
use Doctrine\Common\DataFixtures\OrderedFixtureInterface;
use Symfony\Component\DependencyInjection\ContainerAwareInterface;
use Symfony\Component\DependencyInjection\ContainerInterface;
use Doctrine\Common\Persistence\ObjectManager;
use Symfony\Component\HttpFoundation\File\File;
use Application\Sonata\ClassificationBundle\Entity\Context;
use Application\Sonata\ClassificationBundle\Entity\Category;
class LoadMediaBundleMediaData extends AbstractFixture implements OrderedFixtureInterface, ContainerAwareInterface
{
/**
* #var ContainerInterface
*/
private $container;
/**
* {#inheritDoc}
*/
public function load(ObjectManager $manager)
{
$context = new Context();
$context->setId('default');
$context->setName('default');
$context->setEnabled(true);
$manager->persist($context);
$manager->flush();
$category = new Category();
$category->setName('Default');
$category->setContext($context);
$category->setEnabled(true);
$manager->persist($category);
$manager->flush();
$media1File = new File(__DIR__ . '/../Assets/Images/image.jpg');
$mediaManager = $this->container->get('sonata.media.manager.media');
$media1 = $mediaManager->create();
$media1->setEnabled(true);
$media1->setBinaryContent($media1File);
$media1->setName('brunnkogel.jpg');
$media1->setContext('default');
$media1->setProviderName('sonata.media.provider.image');
$media1->setProviderStatus(1);
$mediaManager->save($media1);
$manager->persist($media1);
$manager->flush();
}
public function setContainer(ContainerInterface $container = null)
{
$this->container = $container;
}
/**
* {#inheritDoc}
*/
public function getOrder()
{
return 40;
}
}
If I create a media in the admin interface it works and the image is saved at web/uploads/media/default/0001/01/c4412442f8f146db6acaec10751fb54e4c80e8c5.jpeg. Also the thumbnails are in this directory. If I create the media by fixtures, the files in web/uploads/media are missing.
I'm using Symfony 2.8 and SonataMediaBundle 3.0.
How can I solve this issue?