code: public void addFile(File file, Integer order) throws HelloSignException {
          Document doc = new Document();
          doc.setFile(file);
          if (order == null) {
              addDocument(doc);
          } else {
              addDocument(doc, order);
          }
      }
docs: Adds the file to the request in the given order.
  
  The order should be a 0-based index into the file list. Therefore, the
  first item of the file list is 0, and so forth.
  
  If order is null, the document is appended to the end of the file list.
  
  @param file File
  @param order Integer or null
  @throws HelloSignException thrown if there is a problem attaching the
  File to this request.