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);
        }
    }
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 ) ; } }
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.