response.setContentType("application/zip");
response.setHeader("Content-Disposition","inline;filename=xpitest.xpi;");

response.setContentType("application/x-xpinstall");
response.setHeader("Content-Disposition","filename=xpitest.xpi;");

HTTP/1.1 200 OK
Date: Wed, 11 Nov 2009 04:51:03 GMT
Server: Apache
Last-Modified: Thu, 05 Nov 2009 15:10:39 GMT
Accept-Ranges: bytes
Content-Length: 4248
Keep-Alive: timeout=2, max=100
Connection: Keep-Alive
Content-Type: application/x-xpinstall