Can't get thumbnails

Hi,
thanks for your work, it seems awesome.
Please can you help me get thumbnails of the files.

I'm trying to list the contents of the root:
`$list = $client->getRoot()->getChildren();
foreach($list as $file) {
      var_dump($file)
}`

Unfrotunately I can't get any thumbnail.
I've tried to specify some $args to the `getChildren()` methd with no luck.

Thanks for any help.