找到解决方法了：
1、先手动把文件上传到云储存，然后获取到fileId
2、调用cloud.downloadFile把文件下载下来获取到文件内容(Buffer类型)
3、调用cloud.openapi.customerServiceMessage.uploadTempMedia把文件上传到微信服务器，获取到mediaId，接下来就可以用这个mediaId发送图片客服消息了

//关注码的fileId
const qrCodeFileId = "cloud://xxxxxxxxx.jpg";
const result = await cloud.openapi.customerServiceMessage.send({
    touser: openid,
    msgtype: 'image',
    image: {
        mediaId: await uploadTmpMedia(qrCodeFileId)
    }
});
console.log(result);

/**
 * 上传临时素材
 * @param fileId
 * @returns {Promise<*>}
 */
const uploadTmpMedia = async function (fileId) {
    const res = await cloud.downloadFile({
        fileID: fileId,
    });
    const buffer = res.fileContent;
    const uploadResult = await cloud.openapi.customerServiceMessage.uploadTempMedia({
        type: 'image',
        media: {
            contentType: 'image/png',
            value: buffer
        }
    });
    console.log(uploadResult);
    return uploadResult.mediaId;
};