<form action ="http://localhost/test.php" method = POST> 
 <input type = text name = uname> 
 <input type = password name =passwd> 
 <input type = FILE name = uploadfile> 
 <input type=submit> 
</form> 

public static string UploadFileEx( string uploadfile, string url,  
  string fileFormName, string contenttype,NameValueCollection querystring,  
  CookieContainer cookies)  
{  
  if( (fileFormName== null) ||  
    (fileFormName.Length ==0))  
  {  
    fileFormName = "file";  
  }  
 
  if( (contenttype== null) ||  
    (contenttype.Length ==0))  
  {  
    contenttype = "application/octet-stream";  
  }  
   string postdata;  
  postdata = "?";  
  if (querystring!=null)  
  {  
    foreach(string key in querystring.Keys)  
    {  
      postdata+= key +"=" + querystring.Get(key)+"&";  
    }  
  }  
  Uri uri = new Uri(url+postdata);  

  string boundary = "----------" + DateTime.Now.Ticks.ToString("x");  
  HttpWebRequest webrequest = (HttpWebRequest)WebRequest.Create(uri);  
  webrequest.CookieContainer = cookies;  
  webrequest.ContentType = "multipart/form-data; boundary=" + boundary;  
  webrequest.Method = "POST";  

  // Build up the post message header  
  StringBuilder sb = new StringBuilder();  
  sb.Append("--");  
  sb.Append(boundary);  
  sb.Append("");  
  sb.Append("Content-Disposition: form-data; name=\"");  
  sb.Append(fileFormName);  
  sb.Append("\"; filename=\"");  
  sb.Append(Path.GetFileName(uploadfile));  
  sb.Append("\"");  
  sb.Append("");  
  sb.Append("Content-Type: ");  
  sb.Append(contenttype);  
  sb.Append("");  
  sb.Append("");        
 
  string postHeader = sb.ToString();  
  byte[] postHeaderBytes = Encoding.UTF8.GetBytes(postHeader);  
 
  // Build the trailing boundary string as a byte array  
  // ensuring the boundary appears on a line by itself  
  byte[] boundaryBytes =  
      Encoding.ASCII.GetBytes("--" + boundary + "");  
 
  FileStream fileStream = new FileStream(uploadfile,  
                FileMode.Open, FileAccess.Read);  
  long length = postHeaderBytes.Length + fileStream.Length +  
                      boundaryBytes.Length;  
  webrequest.ContentLength = length;  
 
  Stream requestStream = webrequest.GetRequestStream();  
 
  // Write out our post header  
  requestStream.Write(postHeaderBytes, 0, postHeaderBytes.Length);  
 
  // Write out the file contents  
  byte[] buffer = new Byte[checked((uint)Math.Min(4096,  
               (int)fileStream.Length))];  
  int bytesRead = 0;  
  while ( (bytesRead = fileStream.Read(buffer, 0, buffer.Length)) != 0 )  
    requestStream.Write(buffer, 0, bytesRead);  
 
  // Write out the trailing boundary  
  requestStream.Write(boundaryBytes, 0, boundaryBytes.Length);  
  WebResponse responce = webrequest.GetResponse();  
  Stream s = responce.GetResponseStream();  
  StreamReader sr = new StreamReader(s);  
 
  return sr.ReadToEnd();  
}

CookieContainer cookies = new CookieContainer();  
//add or use cookies  
NameValueCollection querystring = new NameValueCollection();  
querystring["uname"]="uname";  
querystring["passwd"]="snake3";  
string uploadfile;// set to file to upload  
uploadfile = "c:\\test.jpg";  
 
//everything except upload file and url can be left blank if needed  
string outdata = UploadFileEx(uploadfile,  
   "http://localhost/test.php","uploadfile", "image/pjpeg",  
   querystring,cookies);