添加依赖

implementation 'io.reactivex.rxjava2:rxjava:2.2.10'
implementation 'io.reactivex.rxjava2:rxandroid:2.1.1'
implementation 'com.squareup.retrofit2:converter-gson:2.6.0'
implementation 'com.squareup.retrofit2:adapter-rxjava2:2.6.0'

接口文档

参见：https://sm.ms/doc/

第1步：创建Retrofit请求接口

public interface MainService {
    @Multipart
    @POST("https://sm.ms/api/upload")
    @Headers({"User-Agent: Mozilla/5.0 (Windows NT 6.1; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/75.0.3770.100 Safari/537.36"})
    Observable<Result> uploadImage(@Part MultipartBody.Part filePart, @Part("format") RequestBody format);
}

第2步：使用Retrofit请求网络

/**
 * 使用Retrofit上传图片
 */
public void uploadImageByRetrofit(String path) throws Exception {
    // 创建Retrofit对象
    Retrofit retrofit = new Retrofit.Builder()
            .baseUrl("https://sm.ms/")
            .addConverterFactory(GsonConverterFactory.create())
            .addCallAdapterFactory(RxJava2CallAdapterFactory.create())
            .build();
    // 构建第一个请求参数
    MainService service = retrofit.create(MainService.class);
    // 构建接口请求参数
    MediaType fileType = MediaType.parse("application/octet-stream");
    File file = new File(path); // 要上传的文件对象
    // 文件名中包含中文时，必须进行URL编码
    String fileName = URLEncoder.encode(file.getName(), "UTF-8");
    RequestBody requestBody = RequestBody.create(fileType, file);
    MultipartBody.Part filePart = MultipartBody.Part.createFormData("smfile", fileName, requestBody);
    // 构建第二个请求参数
    MediaType formatType = MediaType.parse("text/plain");
    RequestBody format = RequestBody.create(formatType, "json");
    // 开始请求服务器
    Observable<Result> observable = service.uploadImage(filePart, format);
    observable.subscribeOn(Schedulers.io())
            .observeOn(AndroidSchedulers.mainThread())
            .subscribe(new Observer<Result>() {
                @Override
                public void onSubscribe(Disposable d) {
                    // 开始请求
                }

                @Override
                public void onNext(Result result) {
                    // 请求成功
                    System.out.println("onNext "+result.getData().getUrl());
                }

                @Override
                public void onError(Throwable e) {
                    // 请求失败
                    System.out.println("onError " + e.getMessage());
                }

                @Override
                public void onComplete() {

                }
            });
}