--MIMEBoundary_199ca6b7114b9acca5deb2047d25d5841d4afb7f68281379
Content-Type: application/xop+xml; charset=utf-8; type="text/xml"
Content-Transfer-Encoding: binary
Content-ID: <0.099ca6b7114b9acca5deb2047d25d5841d4afb7f68281379@apache.org>

<?xml version="1.0" encoding="utf-8"?><soapenv:Envelope xmlns:soapenv="http://schemas.xmlsoap.org/soap/envelope/"><soapenv:Header><StateHeader xmlns="http://www.statemef.com/StateGatewayService"><MessageID>12345201704200009962</MessageID><RelatesTo>12345201704200009962</RelatesTo><Action>GetNewAcks</Action><Timestamp>2017-02-11T01:54:51.676-05:00</Timestamp><TestIndicator>T</TestIndicator></StateHeader></soapenv:Header><soapenv:Body><GetNewAcksResponse xmlns="http://www.statemef.com/StateGatewayService"><MoreAvailable>true</MoreAvailable><AcknowledgementListAttachmentMTOM><xop:Include xmlns:xop="http://www.w3.org/2004/08/xop/include" href="cid:299ca6b7114b9acca5deb2047d25d5841d4afb7f68281379@apache.org"></xop:Include></AcknowledgementListAttachmentMTOM></GetNewAcksResponse></soapenv:Body></soapenv:Envelope>
--MIMEBoundary_199ca6b7114b9acca5deb2047d25d5841d4afb7f68281379
Content-Type: application/octet-stream
Content-Transfer-Encoding: binary
Content-ID: <299ca6b7114b9acca5deb2047d25d5841d4afb7f68281379@apache.org>

public static MimeEntity Load (ParserOptions options, ContentType contentType, Stream content, CancellationToken cancellationToken = default (CancellationToken));

public static MimeEntity Load (ContentType contentType, Stream content, CancellationToken cancellationToken = default (CancellationToken));

MimeEntity ParseMultipartFormData (HttpWebResponse response)
{
    var contentType = ContentType.Parse (response.ContentType);

    return MimeEntity.Parse (contentType, response.GetResponseStream ());
}

int i = 1;
foreach (var attachment in multipart.OfType<MimePart> ()) {
    string fileName = string.Format ("attachment.{0}.dat", i++);
    using (var stream = File.Create (fileName))
        attachment.ContentObject.DecodeTo (stream);
}

HttpWebRequest request = CreateWebRequestGetAcks();
                CookieContainer myContainer = new CookieContainer();
                request.PreAuthenticate = true;
                request.CookieContainer = myContainer;
                String username = user;
                String password = pwd;
                String encoded = System.Convert.ToBase64String(System.Text.Encoding.GetEncoding("ISO-8859-1").GetBytes(username + ":" + password));
                request.Credentials = CredentialCache.DefaultCredentials;
                request.Headers.Add("Authorization", " Basic " + encoded);
                 using (Stream stream = request.GetRequestStream())
                {
                    soapEnvelopeXml.Save(stream);
                }
                using (WebResponse response = request.GetResponse())
                {
                using (StreamReader rd = new StreamReader(response.GetResponseStream()))
                    {
                        string _soapResult = rd.ReadToEnd();
                    }
                }
    public static HttpWebRequest CreateWebRequestGetAcks()
                    {
                        HttpWebRequest webRequest = (HttpWebRequest)WebRequest.Create("https://xxxxxxxxxx");
                        webRequest.Headers.Add(@"SOAP:Action");
                        webRequest.ContentType = "text/xml;charset="utf-8"";
                        webRequest.Host = "some thing";
                        webRequest.UserAgent = "Mozilla/4.0 (compatible; MSIE 6.0; MS Web Services Client Protocol 4.0.30319.34209)";
                        webRequest.Method = "POST";`
                        return webRequest;


        }

MIME-Version: 1.0 content-type: multipart/related; type="application/xop+xml";start="<http://tempuri.org/0>";boundary="MIMEBoundary_someuniqueID";start-info="text/xml"
Server: Microsoft-IIS/10.0
X-Powered-By: ASP.NET
Content-Length:24371900


--MIMEBoundary_someuniqueID
Content-Type: application/xop+xml; charset=utf-8; type="text/xml" Content-Transfer-Encoding: binary
Content-ID: <http://tempuri.org/0>

<soap:Envelope>
    <someWrapperElt>
        <xop:Include href="cid:uri_of_content"></xop:Include>
    </someWrapperElt>
</soap:Envelope>
--MIMEBoundary_someuniqueID
Content-Type: application/octet-stream
Content-Transfer-Encoding: binary
Content-ID: <uri_of_content>

...start.b1n@ry-content-here-etc.fckZ8990832d...
--MIMEBoundary_someuniqueID

XmlDictionaryReader mtomReader = XmlDictionaryReader.CreateMtomReader(ms, Encoding.UTF8, XmlDictionaryReaderQuotas.Max);