#default_value not working for file field type

According to the api docs, #default_value is a supported property of the file form control.

However this isn't working in my testing. Example:

  $form[$fieldname] = array(
    '#type' => 'file',
    '#title' => t($field->label),
    '#description' => ($node->$fieldname ? t('"%filename" has been uploaded. If you upload another file, the current file will be replaced.', array('%filename' => $node->$fieldname->filename)) : '') . t($field->description),
    '#required' => $field->required,
    '#weight' => $field->weight,
    '#default_value' => 'TESTING.txt',
    );