File Uploads with Angular and Express

bindCallback.ts

from(files)
  .pipe(
    mergeMap(selectedFile => {
      const fileEntry = selectedFile.fileEntry as FileSystemFileEntry;
      const observableFactory = bindCallback(fileEntry.file) as any;
      const file$ = observableFactory.call(fileEntry) as Observable<File>;
      return file$;
    })
  );