title: TYPO3 - Add flexform to my own extension
body: <p>I am building my own extension.
  I have found this page about adding Flexform to the Extension
  <a href="https://gist.github.com/alrnz/c0f00b196d378f5b9150" rel="nofollow">https://gist.github.com/alrnz/c0f00b196d378f5b9150</a></p>
  
  <p>And in my ext_tables.php i have this:</p>
  
  <pre><code>\TYPO3\CMS\Extbase\Utility\ExtensionUtility::registerPlugin(
      $_EXTKEY,
      'Xlsxtotables',
      'XLSX to tables'
  );
  
  // Include flex forms
  $pluginSignature = str_replace('_', '', $_EXTKEY) . '_' .    'xlsxtotables'; // from registerPlugin(...)
  $TCA['tt_content']['types']['list']['subtypes_addlist']   [$pluginSignature] = 'pi_flexform';
  \TYPO3\CMS\Core\Utility\ExtensionManagementUtility::addPiFlexFormValue(
      $pluginSignature,    
      'FILE:EXT:' . $_EXTKEY . '/Configuration/FlexForms/flexform_xlsxtotables.xml'
  ); 
  </code></pre>
  
  <p>I know that the XML-file is in the right place, but I do not get anyting from it in TYPO3 backend.</p>
  
  <p>Any suggestions?</p>