docs: public static synchronized FileTransferNegotiator getInstanceFor(
              final XMPPConnection connection) {
          FileTransferNegotiator fileTransferNegotiator = INSTANCES.get(connection);
          if (fileTransferNegotiator == null) {
              fileTransferNegotiator = new FileTransferNegotiator(connection);
              INSTANCES.put(connection, fileTransferNegotiator);
          }
          return fileTransferNegotiator;
      }