将多种形式的cURL请求转换为C#post web请求

在Curl中给出了这个多部分表单POST请求

curl -u user:pass -H "Expect: " -F "file=@"C:'Users'Desktop'45.gpx -F description=test -F tags=xxxx -F visibility=xxxx http://www.openstreetmap/api/0.6/gpx/create

我一直试图将其转换为C#，但不知怎么的，我一直收到一个错误的请求400。

System.Collections.Specialized.NameValueCollection reqparm = new System.Collections.Specialized.NameValueCollection();
        reqparm.Add("description", "test");
        reqparm.Add("tags", "upload");
        reqparm.Add("visibility", "public");
        reqparm.Add("file", fileName);
    using (WebClient client = new WebClient())
    {
        try
        {
            client.Credentials = new NetworkCredential("user", "pass");
            //client.Headers.Add("description", "test");
            //client.Headers.Add("tags", "upload");
            //client.Headers.Add("visibility", "public");
            //client.Headers.Add("file", fileName);
            byte[] responsebytes = client.UploadValues(url, "POST",reqparm);
            string responsebody = Encoding.UTF8.GetString(responsebytes);
            Console.WriteLine(responsebody);

        }

        catch (WebException ex)
        {

            Console.WriteLine(ex.Message);
        }
    }