Send Soap Request with Attachment using C#

I want to send soap request with attachment in asp.net.
My original soap request is like below:

POST /SendMessageService/services/SendMessage HTTP/1.1 Accept-Encoding: gzip,deflate SOAPAction: "" Content-Type: multipart/related; type="text/xml"; start="<rootpart@soapui.org>"; boundary="----=_Part_21_19233785.1418547486693" MIME-Version: 1.0 User-Agent: Jakarta Commons-HttpClient/3.1 Host: 10.157.79.79:8310 Content-Length: 8465 ------=_Part_21_19233785.1418547486693 Content-Type: text/xml; charset=UTF-8 Content-Transfer-Encoding: 8bit Content-ID: <rootpart@soapui.org> <soapenv:Envelope xmlns:soapenv="http://schemas.xmlsoap.org/soap/envelope/" xmlns:v2="http://www.huawei.com.cn/schema/common/v2_1" xmlns:loc="http://www.csapi.org/schema/parlayx/multimedia_messaging/send/v2_4/local"> <soapenv:Header> ...