Attaching a system js library using #attached property (D7) When defining a form using the form api I can attach my own css and js like this:

$form['#attached']['css'] = array(
    drupal_get_path('module', 'my_module') . '/my_css_file.css',
);
$form['#attached']['js'] = array(
    drupal_get_path('module', 'my_module') . '/my_js_file.js',
);


This page on Drupal.org suggests that we can attach system libraries the same way and gives this example:

$form['#attached']['library'][] = array('system', 'jquery.cookie');


However when I add this line to my form:

$form['#attached']['library'][] = array('ui.droppable');


Then the library does not get added.

If I go with the old method of:

drupal_add_library('system', 'ui.droppable');


Then is does get added.

What am I doing wrong?
Going from the working example, I imagine this is what you need:

$form['#attached']['library'][] = array('system', 'ui.droppable');


Without the module name Drupal won't know where to find the library hooks.

But if you want to add a custom library from sites/all/libraries/myAwesomeLibrary this is the right way:

$form['#attached']['libraries_load'][] = array('myAwesomeLibrary');