附件在 Slack Block Kit Builder 中不可见

问：

我几乎从 Slack 文档中复制了它，但我在 Block Kit Builder 中看不到附件部分。我错过了什么？

{
    "blocks": [
        {
            "type": "section",
            "text": {
                "type": "mrkdwn",
                "text": "bar"
            }
        }
    ],
    "attachments": [
        {
            "fallback": "Plain-text summary of the attachment.",
            "color": "#2eb886",
            "pretext": "Optional text that appears above the attachment block",
            "author_name": "Bobby Tables",
            "author_link": "http://flickr.com/bobby/",
            "author_icon": "http://flickr.com/icons/bobby.jpg",
            "title": "Slack API Documentation",
            "title_link": "https://api.slack.com/",
            "text": "Optional text that appears within the attachment",
            "fields": [
                {
                    "title": "Priority",
                    "value": "High",
                    "short": false
                }
            ],
            "image_url": "http://my-website.com/path/to/image.jpg",
            "thumb_url": "http://example.com/path/to/thumb.png",
            "footer": "Slack API",
            "footer_icon": "https://platform.slack-edge.com/img/default_application_icon.png",
            "ts": 123456789,
            "blocks": {
                "type": "text",
                "text": "test"
            }
        }
    ]
}