Luma

 protected function getTmpFileName()
{
    $tmpName = null;
    if (isset($_FILES['groups'])) {
        $tmpName = $_FILES['groups']['tmp_name'][$this->getGroupId()]['fields'][$this->getField()]['value'];
    } else {
        $tmpName = is_array($this->getValue()) ? $this->getValue()['tmp_name'] : null;
    }
    return $tmpName;
}