PayPal\Api\TemplateData::addAttachment PHP Method

    public function addAttachment($fileAttachment)
    {
        if (!$this->getAttachments()) {
            return $this->setAttachments(array($fileAttachment));
        } else {
            return $this->setAttachments(array_merge($this->getAttachments(), array($fileAttachment)));
        }
    }