Add category records to TYPO3 file record (e.g. for using it when fetching files via DataProcessing)

AddCategoriesToFile.php

<?php
namespace Vendor\Package\EventListener;

/*
 * This file is part of a TYPO3 extension.
 *
 * It is free software; you can redistribute it and/or modify it under
 * the terms of the GNU General Public License, either version 2
 * of the License, or any later version.
 *
 * For the full copyright and license information, please read the
 * LICENSE.txt file that was distributed with this source code.
 *
 * The TYPO3 project - inspiring people to share!
 */

use TYPO3\CMS\Core\Database\ConnectionPool;
use TYPO3\CMS\Core\Utility\GeneralUtility;

class AddCategoriesToFile
{
    public function __invoke(\TYPO3\CMS\Core\Resource\Event\EnrichFileMetaDataEvent $event): void
    {
        $record = $event->getRecord();
        if ($record['categories'] > 0) {
            $record['categoryRecords'] = $this->getCategories($event->getMetaDataUid());
        }
        $event->setRecord($record);
    }

    protected function getCategories(int $metaDataUid): ?array
    {
        $queryBuilder = GeneralUtility::makeInstance(ConnectionPool::class)->getQueryBuilderForTable('sys_file_metadata');

        return $queryBuilder
            ->select('*')
            ->from('sys_category', 'cat')
            ->leftJoin('cat', 'sys_category_record_mm', 'catmm', 'catmm.uid_local = cat.uid')
            ->where(
                $queryBuilder->expr()->eq('catmm.uid_foreign', $metaDataUid)
            )
            ->execute()
            ->fetchAll();
    }
}

ContentElementWithFile.typoscript

tt_content {
  own_element =< lib.contentElement
  own_element {
    templateName = OwnElement
    dataProcessing {
      10 = TYPO3\CMS\Frontend\DataProcessing\FilesProcessor
      10 {
        references.fieldName = assets
        as = file
      }
    }
  }
}

OwnElement.html

<!-- Output a list of all categories -->
<f:if condition="{file.0.properties.categoryRecords}">
  <f:for each="{file.0.properties.categoryRecords}" as="category" iteration="it">
    <f:if condition="{it.isFirst}"><f:else>, </f:else></f:if>{category.title}
  </f:for>
</f:if>

Services.yaml

services:
  Vendor\Package\EventListener\AddCategoriesToFile:
    tags:
      - name: event.listener
        identifier: 'AddCategoriesToFile'
        event: TYPO3\CMS\Core\Resource\Event\EnrichFileMetaDataEvent
        after: 'languageAndWorkspaceOverlay'

Here a full example for TYPO3 8 with language and workspace overlay:
https://gist.github.com/t3easy/1618ced1d08c6609557b070a0763f318