Uploading a file using an API

Question

I want to upload a file (continuously, not chunked) using the site's API.

Here are the information of the API:

Request to:        http://server/upv3.php
Required Vars:
[username]         account username
[password]         account password
[upload_session]   upload session name from step 1 // I got that
[chunk_no]         set to "1"
[chunk_number]     set to "1"
[filesize]         filesize of current file in bytes
[fn]               filedata (in HTTP forms known as type="file")
[finalize]         set to "1"

Response:          DOWNLOAD-URL;FILESIZE;MD5

Example:           http://www.url.com/dl/HJD74ZDM1;
                   6547231;316508123e89909723fe95945caf00a5
                   (linebreak only in example!)

I think all this properties should be send using POST. The Attribute upload_session is a property I can request successfully.

Here's my implementation:

public void SOUpload()
{
    string soUploadSession = "";
    string soUploadServer = "";

    SOCreateUploadSession(ref soUploadSession, ref soUploadServer);

    HttpWebRequest request = (HttpWebRequest)(HttpWebRequest.Create("http://" + soUploadServer));
    request.Method = "POST";
    request.ContentType = "application/x-www-form-urlencoded";

    long bytesRead = 0;
    byte[] fileBuffer = new byte[file.fileSize];
    FileStream fileStream = new FileStream("C:\\Users\\Patrick\\Downloads\\Prog\\UnityAssets\\start.unitypackage", FileMode.Open);
    byte[] postData = Encoding.ASCII.GetBytes("&username=USERNAME" + "&password=PASSWORD" +
                                              "&upload_session=" + soUploadSession +
                                              "&chunk_no=1"  + "&chunk_number=1"  +
                                              "&filesize=" + file.fileSize.ToString() +
                                              "&finalize=1" + "&fn=");
    Stream requestStream = request.GetRequestStream();
    requestStream.Write(postData, 0, postData.Length);
    while ((bytesRead = fileStream.Read(fileBuffer, 0, fileBuffer.Length)) != 0)
    {
        requestStream.Write(fileBuffer, 0, (int)bytesRead);
    }
    fileStream.Close();
    requestStream.Close();
    Stream responseStream = request.GetResponse().GetResponseStream();
    byte[] responseBuffer = new byte[1024];
    responseStream.Read(responseBuffer, 0, responseBuffer.Length);
    string response = Encoding.ASCII.GetString(responseBuffer);
}

I know how I can implement a download (using octet-stream), so I thought an upload is mostly the same, the main-difference is that instead of reading data from the network-stream and writing this into a file, I am reading data from the file-stream and writing this data to an network-stream....but obviously it does not work that way.