Función para crear un XML Request Object válido

function devuelveXMLRequestObject {
  xmlhttp=false;
  this.requestFile = file;
  this.encodeURIString = true;
  this.execute = false;
    this.AjaxFailedAlert = "Funcionalidades no soportadas.";

  if (window.XMLHttpRequest) {

      this.xmlhttp = new XMLHttpRequest();
      if (this.xmlhttp.overrideMimeType) {
          this.xmlhttp.overrideMimeType('text/xml');
      }
  }
  else if (window.ActiveXObject) { 

  //Internet Explorer

        try {
          this.xmlhttp = new ActiveXObject("Msxml2.XMLHTTP");
      }catch (e) {
          try {
              this.xmlhttp = new ActiveXObject("Microsoft.XMLHTTP");
          } catch (e) {
              this.xmlhttp = null;
          }
      }
      if (!this.xmlhttp && typeof XMLHttpRequest!='undefined') {
          this.xmlhttp = new XMLHttpRequest();
          if (!this.xmlhttp){
              this.failed = true;
          }
      }
  }
  return this.xmlhttp ;
}