将文件发送到Web Api

您好，我在web表单代码后面有一个请求，我喜欢用IFormCollection类型的属性调用web-api-send对象，对象财产正在发送，但文件不是

WebRequest wrqst = WebRequest.Create(URLService + method);
var postData = new StringBuilder();
                foreach (string key in form.Keys)
                {
                    postData.AppendFormat("{0}={1}&",
                        HttpUtility.UrlEncode(key),
                        HttpUtility.UrlEncode(form[key]));
                }

                var data = Encoding.ASCII.GetBytes(postData.ToString());

                wrqst.Method = "POST";
                wrqst.ContentType = "application/x-www-form-urlencoded";
                wrqst.ContentLength = data.Length;

                using (var stream = wrqst.GetRequestStream())
                {
                    stream.Write(data, 0, data.Length);
                }

                
                WebResponse oResponse = wrqst.GetResponse();

我在Request.file中接收文件如何发送文件？