1. In IIS Manager, right-click the website or website directory for which you want to add a MIME type, and click Properties.
2. Click the HTTP Headers tab.
3. Click Mime Types.
4. Click New.
5. In the Extension box, enter the file name extension: dat.
6. In the MIME type box, enter the MIME type description: application/x-ns-proxy-autoconfig.
7. Click OK and then restart the IIS service.

Источник: http://www.sophos.com/support/knowledgebase/article/38784.html

(В некоторых рекомендациях приводится строка AddType application/x-javascript-config dat , ОДНАКО: application/x-ns-proxy-autoconfig will be supported in more clients than application/x-javascript-config)