angular ngrx forkJoin upload multiple

multippleupload.ts

@Effect()
  multiple$ = this.actions.pipe(
    ofType(fromActions.UPLOAD_BILL),
    withLatestFrom(this.store.select(fromStrpGeneralSelectors.selectBuildingId)),
    map(([action, buildingId]): any => {
      return {action, buildingId};
    }),
    mergeMap(({action, buildingId}) => {
      const {files, category} = action.payload;

      const dists = files.map((file) => {
        return this.buildingSettingsService.saveBill(buildingId, {document: file, category: category});
      });

      return forkJoin(dists).pipe(
        map((res: any) => {
          console.log('download completed');
          return new fromActions.UploadBillSuccess(res)
        }),
        catchError(res => of(new fromActions.UploadBillFailure(res.error)))
      );
    })
  );