Multiple file validation: "This value should be of type string"

I'm trying to use Symfony Validator on a file upload form (form extension's validation) and I'm getting this error message:

messageTemplate: "This value should be of type string." from Symfony\Component\Validator\ConstraintViolation

Upload works well without the validator, and I cant figure out where this message is coming from.

Here's my FormType, with a basic validation as doc's exemple:

    {
        $builder
            ->add('file', FileType::class, [
                'label' => 'Choisir un fichier',
                'mapped' => false,
                'multiple' => true,
                'constraints' => [
                    new File([
                        'maxSize' => '1024k',
                        'mimeTypes' => [
                            'application/pdf',
                            'application/x-pdf',
                        ],
                        'mimeTypesMessage' => 'Please upload a valid PDF document',
                    ])
                ],
            ])
        ;
    }

If I remove maxSize, mimeTypes and/or mimeTypesMessage arguments, I still have the same problem.

I can't use annotations on entity (mapped option is set to false).