@POST(URLConstant.URL_PATH)
  Flowable<UploadImgBean> upload(@Body RequestBody Body);

private static RetrofitInterface getRetrofitInterface(String baseUrl) {
    if (retrofitInterface == null) {
      Retrofit retrofit = new Retrofit.Builder()
          .baseUrl(baseUrl)
          .addConverterFactory(GsonConverterFactory.create())
//        .addConverterFactory(ScalarsConverterFactory.create()) //string
          .addCallAdapterFactory(RxJava2CallAdapterFactory.create())
          .build();
      retrofitInterface = retrofit.create(RetrofitInterface.class);
    }
    return retrofitInterface;
  }

 public static Flowable<UploadImgBean> upload(List<File> fileList) {
    //构建body
//addFormDataPart()第一个参数为表单名字，这是和后台约定好的
    MultipartBody.Builder builder = new MultipartBody.Builder()
        .setType(MultipartBody.FORM)
        .addFormDataPart("username", "name") 
        .addFormDataPart("phone", "phone")
   //注意，file是后台约定的参数，如果是多图，file[]，如果是单张图片，file就行
    for (File file : fileList) {
      //这里上传的是多图
      builder.addFormDataPart("file[]", file.getName(), RequestBody.create(MediaType.parse("image/*"), file));
    }
    RequestBody requestBody = builder.build();
    return getRetrofitInterface(URLConstant.URL_BASE).upload(requestBody);
  }

HttpUtils.upload(mUploadFileList)
        .subscribeOn(Schedulers.io())
        .observeOn(AndroidSchedulers.mainThread())
        .subscribe(new DisposableSubscriber<UploadImgBean>() {
          @Override
          public void onNext(UploadImgBean uploadImgBean) {
            Log.i(TAG, "onNext: " ); 
            }
          }

          @Override
          public void onError(Throwable throwable) {
            Log.i(TAG, "onError: --->" + throwable.getMessage());
          }

          @Override
          public void onComplete() {
            Log.i(TAG, "onComplete: ");
          }
        });