Classes/Domain/Model/OverviewNavigationItem.php
public function addThumbnail(\TYPO3\CMS\Extbase\Domain\Model\FileReference $thumbnail) {
    $this->thumbnails->attach($thumbnail);
  }
public function addThumbnail(\TYPO3\CMS\Extbase\Domain\Model\FileReference $thumbnail) {
    $this->thumbnails->attach($thumbnail);
  }
Adds a thumbnail to the overview navigation item.

@param \TYPO3\CMS\Extbase\Domain\Model\FileReference $thumbnail The thumbnail to be added