Add a way to set filename on multipart requests

This has already been discussed in #1063 #1092 #1096 and #1130, but I thought I’d open a separate issue to track this specifically.

RFC2388 describes the filename header parameter of multipart/form-data requests that Retrofit now uses. This parameter is required on some backend configurations. More details and examples can be found here.

We need a way of figuring out what the filename is. The problem with RequestBody (which is a part of OkHttp) is that it doesn’t expose the filename when created with a File.

[wKovacs64]

Using a map works (and may be the best option for beta versions of Retrofit 2 before they brought in OkHttp 3), but I believe they promoted Multipart for exactly this scenario. The current intended way to upload a file while specifying a filename (afaict) is using Multipart.Part. Something like this:

Interface:

@Multipart
@POST("upload")
Call<UploadResponse> uploadImage(@Part MultipartBody.Part filePart)

Client:

RequestBody fileReqBody = RequestBody.create(mediaType, file);
MultipartBody.Part filePart =
    MultipartBody.Part.createFormData(partName, file.getName(), fileReqBody);

Call<UploadResponse> call = api.uploadImage(filePart);

[sbljayarathna]

This is my code. This works…

public interface FileUploadService {  
    @Multipart
    @POST("upload")
    Call<ResponseBody> upload(@PartMap Map<String, RequestBody> params);
}

 private void uploadFile() {  
        FileUploadService service =
                ServiceGenerator.createService(FileUploadService.class);

        File file = new File(selectedImagePath);
image_name = file.getName();
String fileName = "file\"; filename=\"" + image_name;
RequestBody requestBody = RequestBody.create(MediaType.parse("multipart/form-data"), file);

 RequestBody id = RequestBody.create(MediaType.parse("multipart/form-data"), observation_id);

Map<String, RequestBody> requestBodyMap = new HashMap<>();
 requestBodyMap.put("id", id);
requestBodyMap.put(fileName, requestBody);


        Call<ResponseBody> call = service.upload(requestBodyMap);
        call.enqueue(new Callback<ResponseBody>() {
            @Override
            public void onResponse(Call<ResponseBody> call,
                                   Response<ResponseBody> response) {
                Log.v("Upload", "success");
            }

            @Override
            public void onFailure(Call<ResponseBody> call, Throwable t) {
                Log.e("Upload error:", t.getMessage());
            }
        });
    }