Twitter upload/media Append - vb.net

Question

I just can't wrap my mind around how to do the append step. I've already done the INIT part without trouble but I don't understand how to do the next step.
What I'm doing now : I build a authorization header with only the oauth parameters and the signature base too, then I put the required parameters into the body request : command=APPEND, media_id from the INIT method, media with the raw binary image, and segment_index=0 because I have to send only one request since the size of my image is never more than 30 kB.
I have tried the following:
Dim oHttpWebRequest As HttpWebRequest
Dim oHttpWebResponse As HttpWebResponse
Dim oStream As Stream
Dim newLine As String = System.Environment.NewLine
Dim mediaId = getINITresponse()
Try
oHttpWebRequest = DirectCast(WebRequest.Create("https://upload.twitter.com/1.1/media/upload.json"), HttpWebRequest)
Dim sBody As String = buildHeader(twitter, sAuth)
oHttpWebRequest.Headers.Add("Authorization", sBody)
oHttpWebRequest.Method = "POST"
Dim sBoundary As String = DateTime.Now.Ticks.ToString("x")
Dim sStartBoundary As String = "--" + sBoundary + newLine
Dim sEndBoundary As String = newLine + "--" + sBoundary + "--" + newLine
oHttpWebRequest.ContentType = "multipart/form-data; boundary=" + sBoundary
Dim sBodyData As String = ""
Dim srequestString As New StringBuilder()
sBodyData = "command=APPEND&media_id=" + mediaId + "&segment_index=0&media="
srequestString.AppendLine(sStartBoundary + "Content-Disposition: form-data; name=""media""; filename=""image.jpg""" + newLine)
srequestString.AppendLine(Encoding.Default.GetString(getBinaryFileFromPath(sFicName)))
srequestString.AppendLine(newLine + sEndBoundary + newLine)
Dim postData As Byte()
postData = Text.Encoding.UTF8.GetBytes(sBodyData)
Dim postDataBytes As Byte()
postDataBytes = Text.Encoding.UTF8.GetBytes(srequestString.ToString())
Dim byteResult() As Byte = postData.Concat(postDataBytes).ToArray()
oHttpWebRequest.ContentLength = byteResult.Length
Using requestStream = oHttpWebRequest.GetRequestStream()
requestStream.Write(byteResult, 0, byteResult.Length)
requestStream.Close()
End Using
oHttpWebResponse = DirectCast(oHttpWebRequest.GetResponse(), HttpWebResponse)
oStream = oHttpWebResponse.GetResponseStream()
Return New StreamReader(oStream, Encoding.UTF8).ReadToEnd()
Catch wex As WebException 'Exception venant du serveur distant
Return New StreamReader(wex.Response.GetResponseStream()).ReadToEnd()
End Try
My Init part is working and I save the media_id and use it for the Append part. My image is no more than 30 kB so it's not a problem of size.
Twitter respond me with the status : 400 Bad Request.
I don't know what I am doing wrong. Sorry for my bad english!

Answer 1

I find the answer:
sBodyData = "command=APPEND&media_id=" + mediaId + "&segment_index=0&media="
I was making my body data as if I was sending a request with ContentType=application/x-www-form-urlencoded.
So I used both of the contentType in the same request and of course it wasn't working. Now it looks like that:
Dim newLine As String = System.Environment.NewLine
srequestString.AppendLine(sStartBoundary + "Content-Disposition: form-data; name=""command""" + newLine)
srequestString.AppendLine("APPEND")
srequestString.AppendLine(sStartBoundary + "Content-Disposition: form-data; name=""media_id""" + newLine)
srequestString.AppendLine(mediaId)
srequestString.AppendLine(sStartBoundary + "Content-Disposition: form-data; name=""media""" + newLine)
srequestString.AppendLine(Encoding.Default.GetString(getBinaryFileFromPath(sFileName)))
srequestString.AppendLine(sStartBoundary + "Content-Disposition: form-data; name=""segment_index""" + newLine)
srequestString.AppendLine("0")
srequestString.AppendLine(sEndBoundary)