ext_localconf.php

<?php
defined('TYPO3_MODE') or die();

if (TYPO3_MODE === 'BE') {
    /** @var \TYPO3\CMS\Extbase\SignalSlot\Dispatcher $signalSlotDispatcher */
    $signalSlotDispatcher = \TYPO3\CMS\Core\Utility\GeneralUtility::makeInstance(\TYPO3\CMS\Extbase\SignalSlot\Dispatcher::class);

    // Register slot when a file has been added
    $signalSlotDispatcher->connect(
        \TYPO3\CMS\Core\Resource\ResourceStorage::class,
        'postFileAdd',
        \Schmitzal\Tinyimg\Service\CompressImageService::class,
        'initializeCompression',
        true
    );

    // Register slot when a file has been replaced
    $signalSlotDispatcher->connect(
        \TYPO3\CMS\Core\Resource\ResourceStorage::class,
        'postFileReplace',
        \Schmitzal\Tinyimg\Service\CompressImageService::class,
        'initializeCompression',
        true
    );

    // Register command
    $GLOBALS['TYPO3_CONF_VARS']['SC_OPTIONS']['extbase']['commandControllers'][$_EXTKEY] =
        \Schmitzal\Tinyimg\Command\CompressImagesCommandController::class;
}