Error uploading items with Walmart marketplace API feed

Question

I am trying to upload items with Walmart marketplace APIs(Bulk Create Item) but always get a response as 500 Internal server error. Any Ideas? the other Api functions are okay but not this func.. Actually Others are using Method is "application/xml" but this is using "multipart/form-data;"

Note that the XML strings/example in the code below are taken from https://developer.walmartapis.com/#the-item-object and then I changed IDs property.

     public bool RequestUpdateItem()
        {
                    string strBaseUrl = string.Format("https://marketplace.walmartapis.com/v2/feeds?feedType=item");
                    string strRequestMethod = "POST";
                    WalmartSignature Signature = new WalmartSignature(consumerId, privateEncodedStr, strBaseUrl, strRequestMethod);

                    string strSignature = string.Empty;
                    HttpWebRequest httpWebRequest = null;
                    HttpWebResponse httpWebResponse = null;
                    WalmartOrderModel orderModel = new WalmartOrderModel();
                    StringBuilder strBuilder = new StringBuilder();
                    bool bReturnValue = false;
                    try
                    {
                        strSignature = Signature.GetSignature(null);   // null -> generate timestamp
                        httpWebRequest = (HttpWebRequest)WebRequest.Create(strBaseUrl);
                        httpWebRequest.Method = strRequestMethod;
                        httpWebRequest.Accept = "application/xml";
                        httpWebRequest.ContentType = "multipart/form-data;";

                        httpWebRequest.Headers.Add("WM_SVC.NAME", "Walmart Marketplace");
                        httpWebRequest.Headers.Add("WM_SEC.AUTH_SIGNATURE", strSignature);
                        httpWebRequest.Headers.Add("WM_CONSUMER.ID", Signature.ConsumerId);
                        httpWebRequest.Headers.Add("WM_SEC.TIMESTAMP", Signature.TimeStamp);
                        httpWebRequest.Headers.Add("WM_QOS.CORRELATION_ID", Guid.NewGuid().ToString());



                        string strRequestBody =
        @"<?xml version=""1.0"" encoding=""UTF-8""?>
        <MPItemFeed xmlns=""http://walmart.com/"" xmlns:xsi=""http://www.w3.org/2001/XMLSchema-instance"" xsi:schemaLocation=""http://walmart.com/MPItem.xsd"">
        <MPItemFeedHeader>
        <version>2.1</version>
        <requestId>333pecEl55nics</requestId>
        <requestBatchId>333ecElect55cs38</requestBatchId>
        </MPItemFeedHeader>   
        <MPItem>
        <sku>BDK72wsd44f</sku>
        <Product>
        <productName>QVS</productName>
        <longDescription>QVS</longDescription>
        <shelfDescription>QVS</shelfDescription>
        <shortDescription>QVS</shortDescription>
        <mainImage>
        <mainImageUrl>http://images.antonline.com/img-main/500/037229400328.jpg</mainImageUrl>
        </mainImage>
        <productIdentifiers>
        <productIdentifier>
        <productIdType>UPC</productIdType>
        <productId>220740171234</productId>
        </productIdentifier>
        </productIdentifiers>
        <productTaxCode>2038710</productTaxCode>
        <additionalProductAttributes>
        <additionalProductAttribute>
        <productAttributeName>product_id_override</productAttributeName>
        <productAttributeValue>true</productAttributeValue>
        </additionalProductAttribute>
        </additionalProductAttributes>
        <Electronics>
        <brand>QVS</brand>
        <ElectronicsCables>
        </ElectronicsCables>
        </Electronics>
        </Product>
        <price>
        <currency>USD</currency>
        <amount>99.29</amount>
        </price>
        <shippingWeight>
        <value>0.12</value>
        <unit>LB</unit>
        </shippingWeight>
        </MPItem>
        </MPItemFeed>";


                        MemoryStream stream = new MemoryStream();
                        StreamWriter writer = new StreamWriter(stream);
                        writer.Write(strRequestBody);
                        writer.Flush();
                        stream.Position = 0;

                        httpWebRequest.ContentLength = stream.Length;


                        using (Stream requestStream = httpWebRequest.GetRequestStream())
                        {
                            stream.CopyTo(requestStream);
                            //requestStream.Write(stream, 0, stream.Length);

                            httpWebResponse = (HttpWebResponse)httpWebRequest.GetResponse();
                            if (httpWebResponse.StatusCode == HttpStatusCode.OK)
                            {
                                Stream streamResponse = httpWebResponse.GetResponseStream();
                                var responseResult = XDocument.Load(streamResponse);
                            }
                        }

                    }
                    catch (Exception ex)
                    {
                        Console.WriteLine("RequestUpdateItem(): " + ex);
                    }

                    return bReturnValue;
                }