gistfile1.txt

public void onResponse(Response<UploadServer> response, Retrofit _retrofit) {
  UploadServer uploadServer = response.body();
  Uri uploadUrl = Uri.parse(uploadServer.uploadUrl);

  RequestBody body = RequestBody.create(MediaType.parse("multipart/form-data"), file);
  Call<SavePhoto> savePhotoCall = vkApi.uploadPhoto(uploadUrl.toString(), body);
}

UploadServer

public class UploadServer {

    @SerializedName("upload_url")
    public String uploadUrl;

    @SerializedName("album_id")
    public long albumId;

    @SerializedName("user_id")
    public long userId;

}

VkApi

public interface VkApi {

    @Multipart
    @POST
    Call<SavePhoto> uploadPhoto(@Url String url,
                                @Part("photo\"; filename=\"image.png\" ") RequestBody _file);

}