在表单数据字段之一是文件的情况下，如何使用retrofit2发出POST请求？

public interface ApiService {
    @Multipart
    @POST("upload")
    Call<ResponseBody> uploadFile(@Part MultipartBody.Part file);
}

Retrofit retrofit = new Retrofit.Builder()
        .baseUrl(BASE_URL)
        .addConverterFactory(GsonConverterFactory.create())
        .build();

ApiService apiService = retrofit.create(ApiService.class);

File file = new File("path/to/file");
RequestBody requestBody = RequestBody.create(MediaType.parse("multipart/form-data"), file);
MultipartBody.Part filePart = MultipartBody.Part.createFormData("file", file.getName(), requestBody);

Call<ResponseBody> call = apiService.uploadFile(filePart);
call.enqueue(new Callback<ResponseBody>() {
    @Override
    public void onResponse(Call<ResponseBody> call, Response<ResponseBody> response) {
        // 处理响应结果
    }

    @Override
    public void onFailure(Call<ResponseBody> call, Throwable t) {
        // 处理请求失败
    }
});