Invoice Logo - The "file_validate_extensions" plugin does not exist.

Ok the way this should be handled is like the following to support earlier versions of Drupal:

  $constraint_definitions = $this->constraintManager->getDefinitions();
    if (isset($constraint_definitions['FileExtension'])) {
      $form['csv']['#upload_validators'] = [
        'FileExtension' => ['extensions' => 'csv'],
      ];
    }
    else {
      $form['csv']['#upload_validators'] = [
        'file_validate_extensions' => ['csv'],
      ];
    }