code: private static boolean byteSizeIsKnown(Collection<? extends UploadItem> uploadItems) {
          for (UploadItem uploadItem : uploadItems) {
              if (uploadItem.getSizeInBytes() == -1) {
                  return false;
              }
          }
          return true;
      }
docs: Determines if there are upload items where the size is
  unknown in advance. This means we cannot predict the total upload size.
  @param uploadItems
  @return