C# 如何向用户发送与他使用VkNet发送给我的附件相同的邮件？

我在聊天机器人中收到一条来自用户的消息，其中包含一个文档形式的附件。我需要识别此文档，转换它并将其发回。但首先，我只需要将相同的文档发送给发件人。我该怎么做

    var msg = Message.FromJson(new VkResponse(updates.Object));

                    try
                    {
                        if (msg.Attachments.Count > 0)
                        {

                             //
                            var attachment = msg.Attachments.First();
                            //


                            if (attachment.Type == typeof(Document))
                            {
                                var doc = attachment.Instance as Document;
                                var attachments = new List<MediaAttachment>{doc};
                                //attachments.Add(attachment);
                                _vkApi.Messages.Send(new MessagesSendParams
                                {
                                    RandomId = new DateTime().Millisecond,
                                    PeerId = msg.PeerId.Value,
                                    Message = "true",
                                    Attachments = attachments


                                });
                                return Ok("ok");
                            }
                            else
                            {
                                _vkApi.Messages.Send(new MessagesSendParams
                                {
                                    RandomId = new DateTime().Millisecond,
                                    PeerId = msg.PeerId.Value,
                                    Message = "false"
                                });
                                return Ok("ok");
                            }
                        }
                    }

[Serializable]
    public sealed class Updates
    {

        [JsonProperty("type")]
        public string Type { get; set; }


        [JsonProperty("object")]
        public JObject Object { get; set; }


        [JsonProperty("group_id")]
        public long GroupId { get; set; }
    }