Categories of files for TYPO3 v8

ext_localconf.php

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

if (TYPO3_MODE === 'FE' && !isset($_REQUEST['eID'])) {
    \TYPO3\CMS\Core\Utility\GeneralUtility::makeInstance(\TYPO3\CMS\Extbase\SignalSlot\Dispatcher::class)->connect(
        \TYPO3\CMS\Core\Resource\Index\MetaDataRepository::class,
        'recordPostRetrieval',
        \Vendor\Extension\Aspect\FileCategoryAspect::class,
        'addCategoriesToMetadata'
    );
}

FileCategoryAspect.php

<?php
namespace Vendor\Extension\Aspect;

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

class FileCategoryAspect
{
    /**
     * Add categories to file metadata
     *
     * @param \ArrayObject $data
     */
    public function addCategoriesToMetadata(\ArrayObject $data)
    {
        if ($data['categories'] > 0) {
            $categories = $this->getCategories($data['uid']);
            $this->languageAndWorkspaceOverlay($categories);
            $data['categories'] = $categories;
        } else {
            $data['categories'] = [];
        }
    }

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

        return $queryBuilder
            ->select('*')
            ->from('sys_category')
            ->join('sys_category',
                'sys_category_record_mm',
                'sys_category_record_mm',
                $queryBuilder->expr()->eq(
                    'sys_category_record_mm.uid_local',
                    'sys_category.uid'
                )
            )
            ->where(
                $queryBuilder->expr()->eq(
                    'sys_category_record_mm.tablenames',
                    $queryBuilder->createNamedParameter('sys_file_metadata', \PDO::PARAM_STR)
                ),
                $queryBuilder->expr()->eq(
                    'sys_category_record_mm.fieldname',
                    $queryBuilder->createNamedParameter('categories', \PDO::PARAM_STR)
                ),
                $queryBuilder->expr()->eq(
                    'sys_category_record_mm.uid_foreign',
                    $queryBuilder->createNamedParameter($metaDataUid, \PDO::PARAM_INT)
                )
            )
            ->execute()
            ->fetchAll();
    }

    /**
     * Do translation and workspace overlay
     *
     * @param array $categories
     */
    public function languageAndWorkspaceOverlay(array &$categories)
    {
        foreach ($categories as $key => $category) {
            $this->getTsfe()->sys_page->versionOL('sys_category', $category);
            $overlaidCategory = $this->getTsfe()->sys_page->getRecordOverlay(
                'sys_category',
                $category,
                $this->getTsfe()->sys_language_content,
                $this->getTsfe()->sys_language_contentOL
            );
            if ($overlaidCategory !== null){
                $categories[$key] = $overlaidCategory;
            }
        }
    }

    /**
     * @return \TYPO3\CMS\Frontend\Controller\TypoScriptFrontendController
     */
    protected function getTsfe()
    {
        return $GLOBALS['TSFE'];
    }
}

MyContentElement.html

<f:if condition="{downloads}">
  <ul>
    <f:groupedFor as="groupedDownloads" groupBy="properties.categories.0.title" each="{downloads}" groupKey="category">
      <li>
        <span>{f:if(condition:'{category}', then: '{category}', else: 'No category')}</span>
        <ul>
          <f:for each="{groupedDownloads}" as="download">
            <li>
              <a title="{f:if(condition: download.title, then: download.title, else: download.name)}" href="{download.publicUrl}" target="_blank">{f:if(condition: download.title, then: download.title, else: download.name)}, {download.size -> f:format.bytes()}</a>
            </li>
          </f:for>
        </ul>
      </li>
    </f:groupedFor>
  </ul>
</f:if>

MyContentElement.typoscript

tt_content.mycontentelement =< lib.contentElement
tt_content.mycontentelement {
  templateName = MyContentElement
  dataProcessing {
    10 = TYPO3\CMS\Frontend\DataProcessing\FilesProcessor
    10 {
      references.fieldName = media
      as = downloads
    }
  }
}