Drupal → Необязательное поле загрузки файлов

/**
 * Form builder.
 */
function mymodule_myform($form, &$form_state) {
  $form['file'] = array(
    '#type' => 'file',
    '#title' => 'File',
  );
 
  $form['submit'] = array(
    '#type' => 'submit',
    '#title' => 'Upload',
  );
 
  return $form;
}
 
/**
 * Validate callback.
 */
function mymodule_myform_validate($form, &$form_state) {
  $file = file_save_upload('file', array(), 'temporary://');
  if ($file !== NULL) {
    if ($file) {
      $form_state['values']['file'] = $file;
    }
    else {
      form_set_error('file', 'File could not be uploaded');
    }
  }
}