Update with Mediaができない

gistfile1.vb

Public Async Function PostMediaAsync(uri As String, param As Dictionary(Of String, String), fileName As String, media As Byte()) As Task(Of HttpResponseMessage)
    Using cli As New HttpClient
        cli.Timeout = TimeSpan.FromSeconds(Timeout)
        If param Is Nothing Then param = New Dictionary(Of String, String)
        Dim boud = System.Environment.TickCount.ToString
        Dim content As New MultipartFormDataContent("--" + boud)
        For Each x In param
            content.Add(New StringContent(x.Value), """" + x.Key + """")
        Next
        Dim binary = New ByteArrayContent(media)
        binary.Headers.ContentType = New Headers.MediaTypeHeaderValue("image/" + fileName.Split("."c).Last.ToLower)
        Await binary.LoadIntoBufferAsync()
        content.Add(binary, """media[]""", """" + fileName + """")
        cli.DefaultRequestHeaders().Authorization = New Headers.AuthenticationHeaderValue("OAuth", CreateAuthorization(New Uri(uri), "POST", param))
        cli.DefaultRequestHeaders.Add("Content-Length", media.Length.ToString)
        'cli.DefaultRequestHeaders.Add("Content-Type", "multipart/form-data; boundary=--" + boud)
        Return Await cli.PostAsync(New Uri(uri), content)
    End Using
End Function