CNM的HttpUrlConnection真他妈难用，狗屎。
网上抄的：

    public static String doPostWithFile(String url,String savefileName,String fileName, String param) {
        String result = "";
        try {
            // 换行符
            final String newLine = "\r\n";
            final String boundaryPrefix = "--";
            // 定义数据分隔线
            String BOUNDARY = "========7d4a6d158c9";
            // 服务器的域名
            URL realurl = new URL(url);
            // 发送POST请求必须设置如下两行
            HttpURLConnection connection = (HttpURLConnection) realurl.openConnection();
            connection.setDoOutput(true);
            connection.setDoInput(true);
            connection.setUseCaches(false);
            connection.setRequestMethod("POST");
            connection.setRequestProperty("Connection","Keep-Alive");
            connection.setRequestProperty("Charset","UTF-8");
            connection.setRequestProperty("Content-Type","multipart/form-data; boundary=" + BOUNDARY);
            // 头
            String boundary = BOUNDARY;
            // 传输内容
            StringBuffer contentBody =new StringBuffer("--" + BOUNDARY);
            // 尾
            String endBoundary ="\r\n--" + boundary + "--\r\n";
            //输出
            OutputStream out = connection.getOutputStream();
            // 1. 处理普通表单域(即形如key = value对)的POST请求（这里也可以循环处理多个字段，或直接给json）
            //这里看过其他的资料，都没有尝试成功是因为下面多给了个Content-Type
            //form-data  这个是form上传 可以模拟任何类型
            contentBody.append("\r\n")
                    .append("Content-Disposition: form-data; name=\"")
                    .append("param" + "\"")
                    .append("\r\n")
                    .append("\r\n")
                    .append(param)
                    .append("\r\n")
                    .append("--")
                    .append(boundary);
            String boundaryMessage1 =contentBody.toString();
            System.out.println(boundaryMessage1);
            out.write(boundaryMessage1.getBytes(StandardCharsets.UTF_8));

            // 2. 处理file文件的POST请求（多个file可以循环处理）
            contentBody = new StringBuffer();
            contentBody.append("\r\n")
                    .append("Content-Disposition:form-data; name=\"")
                    .append("file" +"\"; ")   // form中field的名称
                    .append("filename=\"")
                    .append(fileName +"\"")   //上传文件的文件名，包括目录
                    .append("\r\n")
                    .append("Content-Type:multipart/form-data")
                    .append("\r\n\r\n");
            String boundaryMessage2 = contentBody.toString();
            System.out.println(boundaryMessage2);
            out.write(boundaryMessage2.getBytes("utf-8"));

            // 开始真正向服务器写文件
            File file = new File(savefileName);
            DataInputStream dis= new DataInputStream(new FileInputStream(file));
            int bytes = 0;
            byte[] bufferOut =new byte[(int) file.length()];
            bytes =dis.read(bufferOut);
            out.write(bufferOut,0, bytes);
            dis.close();
            byte[] endData = ("\r\n--" + BOUNDARY + "--\r\n").getBytes();
            out.write(endData);
            out.flush();
            out.close();

            // 4. 从服务器获得回答的内容
            String strLine="";
            String strResponse ="";
            InputStream in =connection.getInputStream();
            BufferedReader reader = new BufferedReader(new InputStreamReader(in));
            while((strLine =reader.readLine()) != null)
            {
                strResponse +=strLine +"\n";
            }
            System.out.print(strResponse);
            return strResponse;
        } catch (Exception e) {
            System.out.println("发送POST请求出现异常！" + e);
            e.printStackTrace();
        }
        return result;
    }