 <p>Have you tried removing the ContentType header and adding it after : </p> <p><a href="http://i2.iis.net/ConfigReference/system.webServer/httpProtocol/customHeaders" rel="nofollow">http://i2.iis.net/ConfigReference/system.webServer/httpProtocol/customHeaders</a></p> <p>I have never done that with app_offline.htm but might work.</p>