Why file extension validator does not work?

Question

Created this based on doc comments: http://api.drupal.org/api/drupal/includes%21file.inc/function/file_validate_extensions/7 My upload field definition:

   $form['xml'] = array(
        #'#title' => t('Upload Document XML'),
        '#type' => 'file',
        '#description' => t('We process only XML files'),
        '#upload_location'    => "public://documents/",
        "#upload_validators"  => array("file_validate_extensions" => array("xml")),

    );

But every file type passed in...

Any suggestions, please?

Am I doing it wrong or I should call file_validate_extensions directly inside validation function