code: 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;
      }
docs: Make upload handler
  
  @return UploadHandler