If you want to request a "POST" method on C#.You should use :
string data;
byte[] buffer = Encoding.UTF8.GetBytes(data);
HttpWebRequest hwr = (HttpWebRequest)WebRequest.Create("http://www.inspire-geoportal.eu/INSPIREValidatorService/resources/validation/inspire");
hwr.Method = "POST";
hwr.ContentType = "application/x-www-form-urlencoded";
hwr.ContentLength = buffer.Length;
Stream PostStream = hwr.GetRequestStream();
PostStream.Write(buffer, 0, buffer.Length);
PostStream.Close();
But i dont know about file code part.