OSS上传文件400错误，提示MalformedPOSTRequest

Unity3D模仿WWW实现的兼容OSS的Multipart上传类
public class MultiPartUpload
{
    public bool isDone;
    public bool isSuccess;
    public string rspString;


    private HttpWebRequest request;
    private HttpWebResponse response;
    private Stream streamResponse;


    private string mUrl;
    private Dictionary<string, string> mKeyValues;
    private string mFileName;
    private byte[] mFileData;
    private byte[] mPostData;
    private string mBoundary;


    private IAsyncResult mResult;


    public MultiPartUpload(string url, Dictionary<string, string> keyValues, string filename, byte[] data)
    {
        streamResponse = null;
        isDone = false;
        isSuccess = false;
        rspString = "";


        mUrl = url;
        mKeyValues = keyValues;
        mFileName = filename;
        mFileData = data;


        mBoundary = "--------9431149156168";


        HttpWebRequest webRequest = (HttpWebRequest)WebRequest.Create(url);
        webRequest.Timeout = -1;
        webRequest.Method = "POST";
        webRequest.ContentType = "multipart/form-data; boundary=" + mBoundary;
        this.request = webRequest;
        mResult = webRequest.BeginGetRequestStream(getRequestStreamCallback, this);
    }


    private void getRequestStreamCallback(IAsyncResult ar)
    {
        try
        {
            Stream postStream = request.EndGetRequestStream(ar);
            string boundary = mBoundary;
            
            string requestBody = "--" + boundary + "\r\n";
            foreach(KeyValuePair<string, string> keyValue in mKeyValues)
            {
                string postField = "Content-Disposition: form-data; name=\"" + keyValue.Key + "\"\r\n"
                    + "\r\n" + keyValue.Value + "\r\n"
                    + "--" + boundary + "\r\n";
                requestBody += postField;
            }
            string tail = "\r\n" + "--" + boundary + "--\r\n";


            List<byte> postData = new List<byte>();
            {
                postData.AddRange(System.Text.Encoding.Default.GetBytes(requestBody));
                string fileString = "Content-Disposition: form-data; name=\"file\"; filename=\"" + mFileName + "\"\r\n\r\n";
                postData.AddRange(System.Text.Encoding.Default.GetBytes(fileString));
                postData.AddRange(mFileData);
                postData.AddRange(System.Text.Encoding.Default.GetBytes(tail));
            }
            mPostData = postData.ToArray();


            postStream.Write(mPostData, 0, mPostData.Length);
            postStream.Close();
            mResult = (IAsyncResult)request.BeginGetResponse(new AsyncCallback(getResponseCallback), this);
        }
        catch (Exception e)
        {
            rspString = e.Message;
            isDone = true;
        }
    }


    private void getResponseCallback(IAsyncResult ar)
    {
        try
        {
            HttpWebResponse rsp = (HttpWebResponse)request.EndGetResponse(ar);
            Stream streamRsp = rsp.GetResponseStream();
            StreamReader streamRead = new StreamReader(streamRsp);


            rspString = streamRead.ReadToEnd();
            isSuccess = rsp.StatusCode == HttpStatusCode.OK;
            isDone = true;


            streamRsp.Close();
            streamRead.Close();
            rsp.Close();
        }
        catch (Exception e)
        {
            rspString = e.Message;
            isDone = true;
        }
    }
}

unity3d中构造的表单每一个表单域都到有content-type，oss不接受这样的请求，所以报错了