$allowed_exts = array('jpg jpeg gif png txt doc xls xlsx pdf ppt pptx pps odt ods odp docx zip rar');

      $form['file1'] = array(
        '#title' => 'Attachments',
        '#description' => t('Allowed Files - jpg jpeg gif png txt doc xls xlsx pdf ppt pptx pps odt ods odp docx zip rar'),
        '#attributes' => array('id' =>'file_attachment'),
        '#type' => 'managed_file',
        '#upload_validators' => array('file_validate_extensions' => $allowed_exts,)
      );