File.php

<?php

/**
 * @Assert\Callback(methods={"validateFile"})
 */
class File
{
    public function validateFile(ExecutionContext $context)
    {
        if (null !== $this->getId()) {
            return;
        }

        if (null === $this->getFile()) {
            $notBlank = new NotBlank();
            $context->addViolationAtSubPath('file', $notBlank->message);
        }
    }
}