code: private String generateMultipartBoundary() {
          byte[] boundary = new byte[this.random.nextInt(11) + 30];
          for (int i = 0; i < boundary.length; i++) {
              boundary[i] = BOUNDARY_CHARS[this.random.nextInt(BOUNDARY_CHARS.length)];
          }
  
          final String constructedBoundry = "----" + new String(boundary, Charsets.US_ASCII);
          return constructedBoundry.toLowerCase();
      }
docs: Generate a multipart boundary.
  <p>The default implementation returns a random boundary.
  Can be overridden in subclasses.