Maknz\Slack\Message::attach PHP Method

    public function attach($attachment)
    {
        if ($attachment instanceof Attachment) {
            $this->attachments[] = $attachment;
            return $this;
        } elseif (is_array($attachment)) {
            $attachmentObject = new Attachment($attachment);
            if (!isset($attachment['mrkdwn_in'])) {
                $attachmentObject->setMarkdownFields($this->getMarkdownInAttachments());
            }
            $this->attachments[] = $attachmentObject;
            return $this;
        }
        throw new InvalidArgumentException('Attachment must be an instance of Maknz\\Slack\\Attachment or a keyed array');
    }