Sending an attachment through SOAP HttpWebRequest

Question

I am attempting to send an attachment through a HttpWebRequest in a Console Application. After a few days of searching and scouring the internet for some understandable assistance on this, I came up with what I think is a decent solution from this site

While I think I've done everything correctly, I am receiving the following error:

Multipart stream ended before a terminating boundary was encountered.

Questions:
I'm hoping to get some assistance/guidance with the multipart error I'm receiving, as well as assistance in attaching the actual byte[] of the XML Document.

Requirements:

1. The data file that needs to be attached is is an XML file which should be an MTOM Attachment. In order to make it Mtom, my understanding is that I need to be sure that the messageEncoding attribute of the <binding> element in the app.config should have a value of "Mtom" and this will be encoded as such.
2. Based on the business requirements (of which is strict), I need to send the byte[] of the file, not simply the contents itself.

Web Request Method

private static HttpWebRequest CreateWebRequest(SoapAction action)
{
    // Retrieve URL from Endpoint in the app.Config based on the action passed into the
    // method.
    string url = GetUrlAddress(action);

    if (url != null)
    {
        HttpWebRequest request = (HttpWebRequest)WebRequest.Create(url);

        request.Headers.Add(HttpRequestHeader.AcceptEncoding, "gzip, deflate");
        request.Headers.Add(HttpRequestHeader.ContentEncoding, "gzip");
        request.Headers.Add("Content-Transfer-Encoding", "8bit");
        request.Headers.Add("SOAPAction", action.ToString());

        request.Method = "POST";

        request.Headers.Add("MIME-Version", "1.0");
        request.ContentType = "multipart/form-data; type=\"application/xop+xml;\" boundary=\"" + BoundaryMarker + "\"";

        request.ClientCertificates.Add(SecurityCertificate.CertificateObject);

        ServicePointManager.Expect100Continue = false;

        return request;
    }
    else
    {
        throw new NullReferenceException("Address for Endpoint could not be resolved.");
    }
}

Method to Submit Request
Based on this post of mine, I believe I am compressing the HttpWebRequest appropriately using GZip.

private static void SubmitRequest(HttpWebRequest request, XDocument soapXml, byte[] formXmlBytes, FileInfo fileToUpload)
{
    using (Stream requestStream = request.GetRequestStream())
    {
        using (GZipStream gz = new GZipStream(requestStream, CompressionMode.Compress, false))
        {
            soapXml.Save(gz);
            WriteToStream(gz, formXmlBytes, fileToUpload.Name);
        }
    }
}

Method used to Write the MIME information and attachment to the Stream

public static void WriteToStream(Stream stream, byte[] formData, string fileName)
    {
        // Write a new line to the stream.
        byte[] newLineBytes = Encoding.UTF8.GetBytes("\r\n");
        stream.Write(newLineBytes, 0, newLineBytes.Length);

        // Write the header to the stream.
        string header = String.Format(HeaderTemplate, BoundaryMarker, fileName, RequestContentID);
        byte[] headerBytes = Encoding.UTF8.GetBytes(header);
        stream.Write(headerBytes, 0, headerBytes.Length);

        // Write a new line to the stream.
        stream.Write(newLineBytes, 0, newLineBytes.Length);

        // Write the formData to the stream.
        stream.Write(formData, 0, formData.Length);

        // Write a new line to the stream.
        stream.Write(newLineBytes, 0, newLineBytes.Length);

        // Write the footer to the stream.
        byte[] boundaryFooterBytes = Encoding.UTF8.GetBytes("--" + BoundaryMarker + "--");
        stream.Write(boundaryFooterBytes, 0, boundaryFooterBytes.Length);
    }

Soap Body Snippet Element
By using Fiddler, I am able to see what the request actually looks like. To me, it appears that the attachment is actually being appended to the request as the XML that it is, instead of (what I thought would be) a byte[].

After this should be the byte[] of the attachment. Currently, the full XML document is displaying.

Accept-Encoding: gzip, deflate
Content-Transfer-Encoding: 8bit
MIME-Version: 1.0
Content-Type: multipart/form-data; type="application/xop+xml;" boundary="--b73acdd180274cab985e4d697bfde428"
Content-Length: 582081
Connection: Keep-Alive

<?xml version="1.0" encoding="utf-8"?>
<soapenv:Envelope xmlns:soapenv="..." ...>
...
<soapenv:Body>
    <urn:Request version="1.0">
        <urn:FileAttachment>cid:b73acdd180274cab985e4d697bfde428</urn:FileAttachment>
    </urn:Request>
</soapenv:Body>
</soapenv:Envelope>
----b73acdd180274cab985e4d697bfde428
Content-Disposition: attachment; filename="test.xml"
Content-Type: text/xml
Content-Id: b73acdd180274cab985e4d697bfde428
<XML OF ATTACHMENT>
----b73acdd180274cab985e4d697bfde428--