Save binary file from html form data with sun.net.httpserver

I am using sun.net.httpserver from JDK to create a simple http service for my device.

Now I want to save the post form data in to a file, but I always found there are additional content saved in the file.

root@android:/sdcard # cat a.jpg
-----------------------------288271103222787
Content-Disposition: form-data; name="myfile"; filename="a.jpg"
Content-Type: text/plain

CONTENT of the uploaded file

-----------------------------288271103222787--

how can I handle this to only save the "CONTENT of the uploaded file"?

My current code is:

private String handleWebSvcApi(HttpExchange exchange, String apiName) throws Exception {
        Log.d(TAG, "handleWebSvcApi : " + apiName);
        InputStream is = exchange.getRequestBody();
        String ret = mWebServiceApiHandler.handleWebServiceApi(apiName, is);
        return ret;
    }

public String handleWebServiceApi(String apiName, InputStream apiData) throws Exception {
        Object ret = null;
        if(apiName.equals("upload")) {
            String filePath = saveApiDataAsFile(apiData);
        }
}

private static String saveApiDataAsFile(InputStream is) throws IOException {
        BufferedInputStream in = new BufferedInputStream(is);
        File outPutFile = new File(Environment.getExternalStorageDirectory().getPath() + "/newfirmware.zip");
        OutputStream os = new FileOutputStream(outPutFile);
        int len = 0;
        byte buff[] = new byte[8192];
        while ((len = in.read(buff)) > 0) {
            os.write(buff, 0, len);
        }
        os.close();
        return outPutFile.getAbsolutePath();
   }