Using multipart/form-data in Windows metro apps - forms

Question

I'm trying to create a way to upload text data (database key) along with an .png image from a Windows 8.1 App, through a multipart/form-data HTTP POST request, to a PHP script hosted on a webserver (which saves the image locally using the database key as a filename)
I've had a look at packet sniffer and it's all coming out the way I think it should, but the problem is that the PHP $_POST value is not passing through and is null on the php script side.
Here is the C# code on the Windows Metro app side:
private async void UploadImage() {
string newLine = Environment.NewLine;
WebRequest request = WebRequest.Create("http://www.mywebsite.com/receiveimage.php");
request.Credentials = CredentialCache.DefaultCredentials;
request.Method = "POST";
string boundary = "---------------------BUIUFBILPQMZ81C12CCC2EVV2RJ";
request.ContentType = "multipart/form-data; boundary=" + boundary;
// Open pre-saved MY_FILE.png file from local disk and store in fh
StorageFolder localFolder = ApplicationData.Current.LocalFolder;
StorageFile fh = await localFolder.GetFileAsync("MY_FILE.png");
// Header strings and values
string Prefix1 = boundary + newLine + "Content-Disposition: form-data; name=\"SERIAL_NO\"" + newLine + newLine; // SERIAL_NO is the php $_POST variable
string Prefix2 = boundary + newLine + "Content-Disposition: form-data; name=\"MY_FILE\"; filename=\"" + "MY_FILE.png\"" + newLine + "Content-Type: application/base64" + newLine + newLine;
byte[] baPrefix1 = System.Text.Encoding.UTF8.GetBytes(Prefix1);
byte[] baValue1 = System.Text.Encoding.UTF8.GetBytes("C4145-12" + newLine); // (C4145-12 is the database key)
IBuffer buffer = await FileIO.ReadBufferAsync(fh);
byte[] baPrefix2 = System.Text.Encoding.UTF8.GetBytes(Prefix1);
byte[] baValue2 = System.Text.Encoding.UTF8.GetBytes(System.Convert.ToBase64String(buffer.ToArray()).Replace("+", "%2B") + newLine);
byte[] combinedData = new byte[baPrefix1.Length + baValue1.Length + baPrefix2.Length + baValue2.Length];
System.Buffer.BlockCopy(baPrefix1, 0, combinedData, 0, baPrefix1.Length);
System.Buffer.BlockCopy(baValue1, 0, combinedData, baPrefix1.Length, baValue1.Length);
System.Buffer.BlockCopy(baPrefix2, 0, combinedData, baPrefix1.Length + baValue1.Length, baPrefix2.Length);
System.Buffer.BlockCopy(baValue2, 0, combinedData, baPrefix1.Length + baValue1.Length + baPrefix2.Length, baValue2.Length);
Stream dataStream = await request.GetRequestStreamAsync();
dataStream.Write(combinedData, 0, combinedData.Length);
dataStream.Dispose();
using (WebResponse response = await request.GetResponseAsync())
{
// "STATUS" is just a text block
STATUS.Text = (((HttpWebResponse)response).StatusDescription);
StreamReader reader = new StreamReader(response.GetResponseStream());
string responseFromServer = reader.ReadToEnd();
STATUS.Text = responseFromServer;
}
}
The PHP script on the server works with other forms but can't recognize the value passed in from this script, so I know it's not an issue with that. I'm convinced it's a problem with the header formatting. I've experimented with the amount of new lines but to no avail. Any ideas?