private static final MediaType MEDIA_TYPE_PNG = MediaType.parse("image/png");

/**
   * 上传多张图片及参数
   * @param reqUrl URL地址
   * @param params 参数
   * @param pic_key 上传图片的关键字
   * @param paths 图片路径
   */
  public Observable<String> sendMultipart(String reqUrl,Map<String, String> params,String pic_key, List<File> files){
    return Observable.create(new Observable.OnSubscribe<String>(){

      @Override
      public void call(Subscriber<? super String> subscriber) {
        MultipartBody.Builder multipartBodyBuilder = new MultipartBody.Builder();
        multipartBodyBuilder.setType(MultipartBody.FORM);
        //遍历map中所有参数到builder
        if (params != null){
          for (String key : params.keySet()) {
            multipartBodyBuilder.addFormDataPart(key, params.get(key));
          }
        }
        //遍历paths中所有图片绝对路径到builder，并约定key如“upload”作为后台接受多张图片的key
        if (files != null){
          for (File file : files) {
            multipartBodyBuilder.addFormDataPart(pic_key, file.getName(), RequestBody.create(MEDIA_TYPE_PNG, file));
          }
        }
        //构建请求体
        RequestBody requestBody = multipartBodyBuilder.build();

        Request.Builder RequestBuilder = new Request.Builder();
        RequestBuilder.url(reqUrl);// 添加URL地址
        RequestBuilder.post(requestBody);
        Request request = RequestBuilder.build();
        mOkHttpClient.newCall(request).enqueue(new Callback() {
          @Override
          public void onFailure(Call call, IOException e) {
            subscriber.onError(e);
            subscriber.onCompleted();
            call.cancel();
          }

          @Override
          public void onResponse(Call call, Response response) throws IOException {
            String str = response.body().string();
            subscriber.onNext(str);
            subscriber.onCompleted();
            call.cancel();
          }
        });
      }
    });
  } 

OkHttp3Utils.getInstance().sendMultipart(Constants.URL.URL_ADD_NOTICE, mMap, "appendix", mImageList)
        .observeOn(AndroidSchedulers.mainThread())
        .subscribeOn(Schedulers.newThread())
        .subscribe(new Subscriber<String>() {
          @Override
          public void onCompleted() {

          }

          @Override
          public void onError(Throwable throwable) {
            LogUtil.i(TAG, "throwable:" + throwable.toString());
          }

          @Override
          public void onNext(String s) {
            
            LogUtil.i(TAG, "s:" + s);
          }
        });