HttpClient PostAsync取消请求

我有一个非常小的例子,我想上传一个文件到一个名为CloudConvert.com

我使用多部分表单来实现这一点,甚至有一个示例说明如何在API控制台上使用html中的多部分表单:

这是他们的例子:

<form action="https://api.cloudconvert.com/convert" method="POST" enctype="multipart/form-data">
<input type="file" name="file">
<input type="hidden" name="apikey" value="banana-my-key">
<input type="hidden" name="inputformat" value="pdf">
<input type="hidden" name="outputformat" value="docx">
<input type="hidden" name="input" value="upload">
<input type="hidden" name="wait" value="true">
<input type="hidden" name="download" value="inline">
<input type="submit" value="Convert!">
</form>    

这是我对应的c代码:

using (var client = new HttpClient())
using (var formData = new MultipartFormDataContent())
{                
    formData.Add(new StreamContent(paramFileStream), "file" );
    formData.Add(new StringContent("banana-my-key"), "apikey" );
    formData.Add(new StringContent("pdf"), "inputformat" );
    formData.Add(new StringContent("docx"), "outputformat" );
    formData.Add(new StringContent("upload"), "input");
    formData.Add(new StringContent("true"), "wait");
    formData.Add(new StringContent("inline"), "download");
    var response = await client.PostAsync("https://api.cloudconvert.com/convert", formData,new CancellationTokenSource(TimeSpan.FromSeconds(2000)).Token);
    if (!response.IsSuccessStatusCode)
    {
        return null;
    }

    return await response.Content.ReadAsStreamAsync();
}

此代码失败,但出现以下异常:

System.Threading.Tasks.TaskCanceledException: The operation was canceled. ---> System.IO.IOException: Unable to read data from the transport connection: Operation canceled. ---> System.Net.Sockets.SocketException: Operation canceled
--- End of inner exception stack trace ---
at System.Net.Sockets.Socket.AwaitableSocketAsyncEventArgs.ThrowException(SocketError error)
at System.Net.Sockets.Socket.AwaitableSocketAsyncEventArgs.GetResult(Int16 token)
at System.Net.Security.SslStreamInternal.<FillBufferAsync>g__InternalFillBufferAsync|38_0[TReadAdapter](TReadAdapter adap, ValueTask`1 task, Int32 min, Int32 initial)
at System.Net.Security.SslStreamInternal.ReadAsyncInternal[TReadAdapter](TReadAdapter adapter, Memory`1 buffer)
at System.Net.Http.HttpConnection

在Cloudconvert中,我可以看到他们报告正在等待上传。

我在这里做错了什么,似乎StreamContent并没有以应有的方式在请求中进行流式传输。

有什么明显的问题吗?

另外,把他们的html例子放进一个html文件,然后用这种方式上传就可以了。