private function addPart(int $structureNo, string $partType)
{
$fields = ['fname', 'pid', 'ftype', 'fsize', 'hasAttach', 'charset'];
$no = isset($this->parts[$partType]['pid']) ? count($this->parts[$partType]['pid']) : 0;
foreach ($fields as $field) {
if (!empty($this->structure[$field][$structureNo])) {
$this->parts[$partType][$field][$no] = $this->structure[$field][$structureNo];
}
}
}