Interpret HTTP (Content-Disposition) headers from an XMLHTTP object?

[evank]

I'm trying to implement an AJAX solution for exporting tabular data on a page in certain formats, and i've determined that the best way to do this (for my purposes) is to grab the innerHTML of a table, send it off to a server-side script for processing into alternate formats, and then returning the response with appropriate Content-Type and Content-Disposition headers. The issue I'm having is how exactly to get the browser to interpret said headers.

For example, I send the table's innerHTML in a POST request to a server-side script (via an XMLHTTP object):

xmlhttp.open("POST", "process.cgi", true);
xmlhttp.onreadystatechange=function() {
  if (xmlhttp.readyState==4) {
    // do what we need to do
  }
}
xmlhttp.send('exportdata='+escape(dataTable.innerHTML));

If someone directly requests the server-side script (ie: through a form), it sends appropriate headers so the user is prompted by the browser to download the file. For example, exporting to an Excel spreadsheet, it sends:

Content-Type: application/vnd.ms-excel
Content-Disposition: attachment; filename="1165420852.xls"

My problem is, I don't know how to get the browser to interpret those headers coming from an XMLHTTP object.