private void okhttpPostMutipleFile() {
   @SuppressLint("SdCardPath") File fileJpg1 = new File("/sdcard/Download/browser/218(1).jpg");
   @SuppressLint("SdCardPath") File fileJpg2 = new File("/sdcard/Download/browser/218(2).jpg");
   @SuppressLint("SdCardPath") File fileJpg3 = new File("/sdcard/Download/browser/218(3).jpg");

    if(!fileJpg1.exists()){
        Log.v(TAG,fileJpg1.getName() + " does not exist");
    }
    else if(!fileJpg2.exists()){
        Log.v(TAG,fileJpg2.getName() + " does not exist");
    }
    else if(!fileJpg3.exists()){
        Log.v(TAG,fileJpg3.getName() + " does not exist");
    }
    else{
        OkHttpClient okHttpClient = new OkHttpClient.Builder()
                .connectTimeout(5000,TimeUnit.MILLISECONDS)
                .build();

        MediaType mediaType = MediaType.parse("image/jpeg");
        RequestBody fileBody1 = RequestBody.create(fileJpg1,mediaType);
        RequestBody fileBody2 = RequestBody.create(fileJpg2,mediaType);
        RequestBody fileBody3 = RequestBody.create(fileJpg3,mediaType);

        // 由于是上传文件，使用的不再是RequestBody.create构建，而是MultipartBody.Builder
        // addFormDataPart有仨参数：文件key值，文件名字,以及文件体
        RequestBody requestBody = new MultipartBody.Builder()
                .addFormDataPart("file",fileJpg1.getName(),fileBody1)
                .addFormDataPart("file",fileJpg2.getName(),fileBody2)
                .addFormDataPart("file",fileJpg3.getName(),fileBody3)
                .build();

        Request request = new Request.Builder()
                .url(BASE_URL + "/files/upload")
                .post(requestBody)
                .build();

        Call postSingleFileTask = okHttpClient.newCall(request);
        postSingleFileTask.enqueue(new Callback() {
            @Override
            public void onFailure(@NotNull Call call, @NotNull IOException e) {
                Log.v(TAG,"Error ==> " + e.toString());
            }

            @Override
            public void onResponse(@NotNull Call call, @NotNull Response response) throws IOException {
                int responseCode = response.code();
                Log.v(TAG,"responseCode ==> " + responseCode);
                if(responseCode == HttpURLConnection.HTTP_OK){
                    String dataReturned = Objects.requireNonNull(response.body()).string();
                    Log.v(TAG,"dataReturned ==> " + dataReturned);
                }
            }
        });
    }
}

Log图片：