Document how to define a variable with a file entity and media element

It took a while before I figured out how to add a multilingual variable which stores a reference to a file entity, selected through the media element (as provided by Media module). Here is my first attempt, based on my experience with Media 7.x-1.3, Media Youtube 7.x-2.0-rc3 and Variable 7.x-2.3.

Example code:

/**
 * Implements hook_variable_info().
 */
function mymodule_variable_info($options) {
  $variables = array();

  // An image.
  $variables['mymodule_image'] = array(
    'type'        => 'array',
    'group'       => 'mymodule_variables',
    'title'       => t('Image', array(), $options),
    'localize'    => TRUE,
    'required'    => TRUE,
    'default'     => array(),
    'element'     => array(
      '#type' => 'media',
      '#media_options' => array(
        'global' => array(
          'types' => array('image'),
          'schemes' => array(),
          'file_extensions' => 'png jpg gif',
        ),
      ),
      '#attributes' => array('class' => array('media-widget')),
    ),
  );

  // A Youtube video.
  $variables['mymodule_youtube_video'] = array(
    'type'        => 'array',
    'group'       => 'mymodule_variables',
    'title'       => t('Youtube video', array(), $options),
    'localize'    => TRUE,
    'required'    => TRUE,
    'default'     => array(),
    'element'     => array(
      '#type' => 'media',
      '#media_options' => array(
        'global' => array(
          'types' => array('video'),
          'schemes' => array('youtube'),
        ),
      ),
      '#attributes' => array('class' => array('media-widget')),
    ),
  );

  return $variables;
}

Note how I explicitly add the 'media-widget' class, which is required for the media selector to work properly. Normally, this is automatically added by the media element, but if you do not add it explicitly in the variable info definition, it gets lost on localizable variables.

Hope this helps anyone.