Set ContentType to text/xml with Javascript

Hi: I have the following Javascript function that's working but only writing the data out as a regular webpage. How do I set the content type to "text/xml" so that it displays as a real xml file?

function openRSS() {
      //var html = '';
      var str = '<?xml version=""1.0"" ?>\n';
            str += '<rss version="2.0">\n';
            str += '<channel>\n';
            //str += '<title></title>\n';
            //str += '<link></link>\n';
            str += '<description>CNIC News</description>\n';
            str += '<language>en-us</language>\n';

         var showTextElem = document.getElementById("txtDiv");

         if (showTextElem != null) {
               str += showTextElem.innerHTML;
         }
         else
         {
            alert("Could not find the text section in the HTML!");
          return;
      }

         //html += '\n</bo' + 'dy>\n</ht' + 'ml>';
      str += '</channel>\n';
      str += '</rss>\n';

         var textWin = window.open("","rss");
         textWin.document.open();
         textWin.document.write(str);
         textWin.document.close();
}