code: 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);
      }
docs: Downloads a thumbnail for the image file at the given path in Dropbox.
  
  @param sizeBound
  The returned thumbnail will never be greater than the dimensions given here.
  
  @param format
  The image format to use for thumbnail data.
  
  @param path
  The path to the image file on Dropbox.
  
  @param rev
  The {@link DbxEntry.File#rev rev} of the file on Dropbox
  or {@code null} if you want the latest revision of the file.
  
  @param target
  The stream to write the thumbnail image data to.
  
  @return
  The metadata for the original file (not the thumbnail) or {@code null} if there
  is no file at that path.