code: public BoxRequestsFile.DownloadThumbnail getDownloadThumbnailRequest(OutputStream outputStream, String fileId) {
          BoxRequestsFile.DownloadThumbnail request = new BoxRequestsFile.DownloadThumbnail(fileId, outputStream, getThumbnailFileDownloadUrl(fileId), mSession);
          return request;
      }
docs: Gets a request that downloads the given file thumbnail to the provided outputStream. Developer is responsible for closing the outputStream provided.
  
  @param outputStream outputStream to write file contents to.
  @param fileId the file id to download.
  @return  request to download a file thumbnail