symfony, cant pass validation on a embedded form + file - forms

Question

Trying to embed a collection or just a simple type containing a file field, but both result in the same problem. On post the UploadedFile is present, but validation fails. "This value is not valid."
Bashing my head in all day, anyone?
Company form:
Simple embed with a file:
$builder->add('logo', new FileType(), [
'label' => 'Logo',
'required' => false,
'attr' => [
'accept' => 'image/*',
]
]);
Collection with files:
$builder->add('images', 'collection', array(
'type' => new FileType(),
'data' => [
new File(),
new File(),
new File(),
],
));
FileType:
class FileType extends AbstractType
{
public function buildForm(FormBuilderInterface $builder, array $options)
{
$data = $builder->getData();
$builder->add('file', 'file', [
'label' => 'Bestand',
]);
$builder->add('submit', 'submit', [
'label' => 'Save',
]);
}
public function getName()
{
return 'file';
}
public function setDefaultOptions(OptionsResolverInterface $resolver)
{
$resolver->setDefaults(array(
'data_class' => 'Acme\DemoBundle\Entity\File',
));
}
}
The entity has a functioniong one-to-many from Company to File.
Everything works, but when the form is submitted. Both fields have a "This value is not valid." error.
Here is a dump of the clientData preSubmit:
'logo' =>
object(Symfony\Component\HttpFoundation\File\UploadedFile)[13]
private 'test' => boolean false
private 'originalName' => string 'etc.jpg' (length=13)
private 'mimeType' => string 'image/jpeg' (length=10)
private 'size' => int 103843
private 'error' => int 0
'images' =>
array (size=3)
0 =>
object(Symfony\Component\HttpFoundation\File\UploadedFile)[14]
private 'test' => boolean false
private 'originalName' => string 'Screen Shot 2014-07-24 at 17.15.30.png' (length=38)
private 'mimeType' => string 'image/png' (length=9)
private 'size' => int 84102
private 'error' => int 0
1 => null
2 => null
Seems perfectly fine to me!