其中的错误原因可能是编码问题。以下示例代码演示了如何将文件上传到服务器，并使用multipart/form-data进行POST：

private void uploadFile(String filePath, String uploadUrl) {
    try {
        // 创建文件
        File file = new File(filePath);
        // 设置请求路径
        URL url = new URL(uploadUrl);
        HttpURLConnection conn = (HttpURLConnection) url.openConnection();
        // 设置请求方式
        conn.setRequestMethod("POST");
        // 设置请求头
        conn.setRequestProperty("Connection", "Keep-Alive");
        conn.setRequestProperty("Content-Type", "multipart/form-data; boundary=" + BOUNDARY);
        // 设置字符集
        conn.setRequestProperty("Charset", "UTF-8");
        // 设置请求超时时间
        conn.setConnectTimeout(15000);
        conn.setReadTimeout(15000);
        // 获取输出流
        OutputStream outStream = conn.getOutputStream();
        // 设置上传参数
        StringBuilder sb = new StringBuilder();
        sb.append("--").append(BOUNDARY).append("\r\n");
        sb.append("Content-Disposition: form-data; name=\"file\"; filename=\"" + file.getName() + "\"\r\n");
        sb.append("Content-Type: application/octet-stream\r\n\r\n");
        outStream.write(sb.toString().getBytes());
        // 读取文件数据
        InputStream inStream = new FileInputStream(file);
        byte[] buffer = new byte[1024];
        int len = 0;
        while ((len = inStream.read(buffer)) != -1) {
            outStream.write(buffer, 0, len);
        }
        inStream.close();
        outStream.write("\r\n".getBytes());
        // 结束标志
        byte[] endData = ("--" + BOUNDARY + "--\r\n").getBytes();
        outStream.write(endData);
        outStream.flush();
        // 获取服务器返回信息
        int res = conn.getResponseCode();
        if (res == 200) {
            InputStream inputStream = conn.getInputStream();
            String result = streamToString(inputStream);
            Log.i(TAG, "uploadFile result: " + result);
        }
        outStream.close();
        conn.disconnect();
    } catch (Exception e) {
        e.printStackTrace();
    }
}

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