code: public Observable<List<Attachment>> uploadAttachments(@NonNull List<Attachment> data, @NonNull RxComapiClient c) {
          if (!data.isEmpty()) {
              return Observable.concatDelayError(upload(c, data)).toList();
          } else {
              return Observable.fromCallable(() -> null);
          }
      }
docs: Create observable to perform attachments upload.
  
  @param data List of Attachments to upload.
  @param c    Comapi client to acces service APIs.
  @return Observable to perform attachments upload.