// Add Context Menu and JS
if (TYPO3_MODE=='BE') {
  $GLOBALS['TYPO3_CONF_VARS']['BE']['ContextMenu']['ItemProviders'][1505197586] = \AR\ArExt\ContextMenu\Mymodule::class;
}
$GLOBALS['TYPO3_CONF_VARS']['SC_OPTIONS']['typo3/backend.php']['constructPostProcess'][] = \AR\ArExt\Hooks\BackendControllerHook::class . '->addJavaScript';

<?php
namespace AR\ArExt\ContextMenu;

class Mymodule extends \TYPO3\CMS\Backend\ContextMenu\ItemProviders\AbstractProvider{

  protected $itemsConfiguration = [
    'filetest' => [
      'type' => 'item',
      'label' => 'File Test Module', // you can use "LLL:" syntax here
      'iconIdentifier' => 'actions-document-open',
      'callbackAction' => 'fileTest' // Hier die fileTest beachten! Diese Funktion taucht unten in der ContextMenuActions.js wieder auf
    ]
  ];

  /**
   * Checks if this provider may be called to provide the list of context menu items for given table.
   *
   * @return bool
   */
  public function canHandle(): bool
  {
    // Current table is: $this->table
    // Current UID is: $this->identifier
    return $this->table === 'sys_file';
  }

  /**
   * Returns the provider priority which is used for determining the order in which providers are processing items
   * to the result array. Highest priority means provider is evaluated first.
   *
   * This item provider should be called after PageProvider which has priority 100.
   *
   * BEWARE: Returned priority should logically not clash with another provider.
   *   Please check @see \TYPO3\CMS\Backend\ContextMenu\ContextMenu::getAvailableProviders() if needed.
   *
   * @return int
   */
  public function getPriority(): int
  {
    return 90;
  }

  /**
   * Registers the additional JavaScript RequireJS callback-module which will allow to display a notification
   * whenever the user tries to click on the "Hello World" item.
   * The method is called from AbstractProvider::prepareItems() for each context menu item.
   *
   * @param string $itemName
   * @return array
   */
  protected function getAdditionalAttributes(string $itemName): array
  {
    return [
      // BEWARE!!! RequireJS MODULES MUST ALWAYS START WITH "TYPO3/CMS/" (and no "Vendor" segment here)
      'data-callback-module' => 'TYPO3/CMS/ArExt/ContextMenuActions',
      // Here you can also add any other useful "data-" attribute you'd like to use in your JavaScript (e.g. localized messages)
    ];
  }

  /**
   * This method adds custom item to list of items generated by item providers with higher priority value (PageProvider)
   * You could also modify existing items here.
   * The new item is added after the 'info' item.
   *
   * @param array $items
   * @return array
   */
  public function addItems(array $items): array
  {

    $this->initDisabledItems();
    $localItems = $this->prepareItems($this->itemsConfiguration);
    $items = $items + $localItems;
    //passes array of items to the next item provider
    return $items;
  }

  /**
   * This method is called for each item this provider adds and checks if given item can be added
   *
   * @param string $itemName
   * @param string $type
   * @return bool
   */
  protected function canRender(string $itemName, string $type): bool
  {
    // checking if item is disabled through TSConfig
    if (in_array($itemName, $this->disabledItems, true)) {
      return false;
    }
    $canRender = false;
    switch ($itemName) {
      case 'filetest':
      $canRender = true;
      break;
      default:
        $canRender = false;
    }
    return $canRender;
  }
}

<?php
namespace AR\ArExt\Hooks;

use TYPO3\CMS\Backend\Controller\BackendController;
use TYPO3\CMS\Backend\Utility\BackendUtility;
use TYPO3\CMS\Core\Page\PageRenderer;
use TYPO3\CMS\Core\Utility\GeneralUtility;

/**
 * This class adds Filelist related JavaScript to the backend
 */
class BackendControllerHook
{
    /**
     * Adds Filelist JavaScript used e.g. by context menu
     *
     * @param array $configuration
     * @param BackendController $backendController
     */
    public function addJavaScript(array $configuration, BackendController $backendController)
    {
        $pageRenderer = GeneralUtility::makeInstance(PageRenderer::class);
        $pageRenderer->addInlineSetting('FileTest', 'moduleUrl', BackendUtility::getModuleUrl('file_test'));
    }
}

/**
 * Module: TYPO3/CMS/ExtensionKey/ContextMenuActions
 *
 * JavaScript to handle the click action of the "Hello World" context menu item
 * @exports TYPO3/CMS/ExtensionKey/ContextMenuActions
 */
define(function () {
  'use strict';

  /**
   * @exports TYPO3/CMS/ExtensionKey/ContextMenuActions
   */
  var ContextMenuActions = {};

  /**
   * Say hello
   *
   * @param {string} table
   * @param {int} uid of the page
   */

    ContextMenuActions.getReturnUrl = function () {
      return top.rawurlencode(top.list_frame.document.location.pathname + top.list_frame.document.location.search);
    };

    ContextMenuActions.fileTest = function (table, uid) { // Hier die fileTest beachten! Diese Funktion haben wir in der ContextMenu Klasse unter callback definiert!
      if (table === 'sys_file') {
        //If needed, you can access other 'data' attributes here from $(this).data('someKey')
        //see item provider getAdditionalAttributes method to see how to pass custom data attributes
        // hier springen wir auf das module FileTest
        top.TYPO3.Backend.ContentContainer.setUrl(
          top.TYPO3.settings.FileTest.moduleUrl + '&target=' + top.rawurlencode(uid) + '&returnUrl=' + ContextMenuActions.getReturnUrl()
        );
      }
    };
    return ContextMenuActions;
});

<?php
use AR\ArExt\Controller;

/**
 * Definitions for routes provided by EXT:backend
 * Contains all "regular" routes for entry points
 *
 * Please note that this setup is preliminary until all core use-cases are set up here.
 * Especially some more properties regarding modules will be added until TYPO3 CMS 7 LTS, and might change.
 *
 * Currently the "access" property is only used so no token creation + validation is made,
 * but will be extended further.
 */
return [
  //
  'file_test' => [
    'path' => '/file/test',
    'target' => Controller\FileTestController::class . '::mainAction'
  ],
];