Make alt text required when uploading images using media widget (Drupal)

gistfile1.txt

/**
 * Implements hook_form_FORM_ID_alter().
 */
function mymodule_form_file_entity_add_upload_alter(&$form) {
  _mymodule_media_form_alter($form);
}

/**
 * Implements hook_form_FORM_ID_alter().
 */
function mymodule_form_file_entity_edit_alter(&$form) {
  _mymodule_media_form_alter($form);
}

/**
 * Alter the title and description of the alt text field and make it required.
 */
function _mymodule_media_form_alter(&$form) {
  $form['field_file_image_alt_text'][LANGUAGE_NONE][0]['value']['#required'] = 1;
  $form['field_file_image_alt_text'][LANGUAGE_NONE][0]['value']['#title'] = t('File description (alt text)');
  $form['field_file_image_alt_text'][LANGUAGE_NONE][0]['value']['#description'] = t('A useful description of the file so it can be found again and is also required for accessibility');
}

In which file(s) would we have to put this code?