mDNS is working with Bonjour

Oh, sorry - that's my mistake copy/pasting. Try:

wifi.enableMDNS = function(hostname, callback) {
  this.at.cmd("AT+MDNS=1,"+JSON.stringify(hostname)+',"http",80\r\n',500,function(d) {
    callback(d=="OK"?null:d);
  });
};