public function getHandler()
    {
        if ($this->namer === null) {
            $this->namer = new GenericNamer();
        }
        if ($this->filesystem === null) {
            throw new \LogicException(sprintf('You should set a filesystem for the builder.'));
        }
        $handler = new UploadHandler($this->filesystem, $this->namer, $this->overwrite);
        $validator = $handler->getValidator();
        foreach ($this->constraints as $constraint) {
            $validator->addConstraint($constraint);
        }
        return $handler;
    }