code: public function getErrorMessage(UploadedFile $file)
      {
          $message = null;
          if ($this->minSize && $this->maxSize) {
              $message = sprintf('File size should between %s and %s', $this->minSize, $this->maxSize);
          } elseif ($this->minSize === null) {
              $message = sprintf('File size should less than %s', $this->maxSize);
          } elseif ($this->maxSize === null) {
              $message = sprintf('File size should greater than %s', $this->minSize);
          }
          return $message;
      }
docs: {@inheritdoc}