docs: public DbxEntry./*@Nullable*/File getThumbnail(
              DbxThumbnailSize sizeBound, DbxThumbnailFormat format,
              String path, /*@Nullable*/String rev, OutputStream target)
              throws DbxException, IOException
      {
          if (target == null) throw new IllegalArgumentException("'target' can't be null");
  
          Downloader downloader = startGetThumbnail(sizeBound, format, path, rev);
          if (downloader == null) return null;
          return downloader.copyBodyAndClose(target);
      }