Update single file in TYPO3 FAL

gistfile1.php

<?php
$storageUid = 0;

$storage = \TYPO3\CMS\Core\Resource\ResourceFactory::getInstance()->getStorageObject($storageUid);
$storage->setEvaluatePermissions(FALSE);

/** @var \TYPO3\CMS\Core\Resource\Index\Indexer $indexer */
$indexer = \TYPO3\CMS\Core\Utility\GeneralUtility::makeInstance('TYPO3\\CMS\\Core\\Resource\\Index\\Indexer', $storage);

$file = $storage->getFile('/uploads/tx_extension/images/image.jpg');
$indexer->updateIndexEntry($file);