Webrequest with form data and file

Hi,

I need to upload a file with a webrequest and combine it with some other form data in the request. Today my post request looks like this:

  Protected Function PostData(ByRef url As String, ByRef POST As String, ByRef Cookie As System.Net.CookieContainer) As String
        Dim request As HttpWebRequest
        Dim response As HttpWebResponse
        request = CType(WebRequest.Create(url), HttpWebRequest)
        request.ContentType = "application/x-www-form-urlencoded"
        request.ContentLength = POST.Length
        request.Method = "POST"
        request.AllowAutoRedirect = False

        Dim requestStream As Stream = request.GetRequestStream()
        Dim postBytes As Byte() = Encoding.ASCII.GetBytes(POST)
        requestStream.Write(postBytes, 0, postBytes.Length)
        requestStream.Close()
        response = CType(request.GetResponse, HttpWebResponse)
        Return New StreamReader(response.GetResponseStream()).ReadToEnd()
    End Function

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The string post (in request.ContentLength)  now contains this form-data:

"access_token=xxyyzzz"

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Now I want also to add this form data parameters:
file (a pdf file to upload)
recipient_email (string value)
recipient_name (string value)

How do I need to alter the PostData function to be able to post both the file and the string data in the webrequest?

Thanks for help!

Peter