我前几天遇到了类似的问题。这里说到我的工作code。

I faced a similar problem a few days ago. Here comes my working code.

首先，你必须定义接口。不添加的内容类型头是非常重要的。
在下面可以看到 @part（图片\\; 图片是对应的名称，后端预期（对我来说），所以你不得不这样做。更改此名称，使其与您的后端工作。文件名= \\图片\\只是定义了你上传的图片的名称。
在我的情况的方法是在DiaryService.java。

At first you have to define the Interface. It is very important NOT to add a content type header. In the following you see the @Part("picture\"; the "picture" is the corresponding name, that the Backend expects (in my case). So you have to change this name to make it work with your Backend. The filename=\"image\" just defines the name of your uploaded image. In my case the method is in the "DiaryService.java".

    @Multipart
    @POST("signature/upload")
    public Call<Void> addImageElementToDiary(@Part("picture\"; filename=\"image\" ") RequestBody picture,
                                             @Part("sort") RequestBody sort);

现在您可以拨打上述定义的服务。重要的是要正确分析的MediaType是非常重要的。你用的multipart / form-data的，这并没有我的情况下工作。您应该在以下Mediatypes看看。

Now you can call the above defined Service. It is important to parse the Mediatype correctly. You used "multipart/form-data", this did not work in my case. You should take a look at the following Mediatypes.

    File file = new File(((CreateDiaryImage) element).getImagePath());

        RequestBody sort = RequestBody.create(MediaType.parse("text/plain"), element.getSort() + "");

        RequestBody requestBody =
                RequestBody.create(MediaType.parse("image/*"), file);

        Call<Void> call = diaryService.addImageElementToDiary(requestBody, sort);
        call.enqueue(new Callback<Void>() {
            @Override
            public void onResponse(Response<Void> response, Retrofit retrofit) {
                Log.v("Upload", "success");
            }

            @Override
            public void onFailure(Throwable t) {
                Log.e("Upload", t.getMessage());
            }
        });