You can use Rebex
HttpRequest class to send "multipart/form-data" requests, which can contain multiple parts using the approach described in answers to this StackOverflow question: https://stackoverflow.com/questions/566462/upload-files-with-httpwebrequest-multipart-form-data
To adapt the code to use Rebex API, replace this line:
HttpWebRequest wr = (HttpWebRequest)WebRequest.Create(url);
var creator = new HttpRequestCreator();
HttpRequest request = creator.Create(url);
However, make sure this is actually the request form expected by the web service you are calling. The web services' documentation should make it clear what kind of requests the HTTP client is supposed to use.