WebDav put of application extensions fails

We have an IIS 6 server running a WebDav site. Users successfully connect and upload most files. The files they cannot upload are the ones that are mapped to extensions, such as PHP or ASP. I checked the verbs and tried adding PUT, but that still didn't allow users to upload those files. They always get back:

Error: 0x80070780: The file cannot be accessed by the system.

When I remove the application mapping, the users can upload the files no problem. What else has to be done to allow application mapped files to be uploaded?

If it's of any use, here is the IIS log everytime someone tries to upload:

2009-06-26 15:44:34 PROPFIND /onpar/test.php - domain\user 192.168.1.1 HTTP/1.1 Microsoft-WebDAV-MiniRedir/6.0.6001 - 403 221

It turns out that in order to upload a file that is mapped to an application extension, you have to enable script source access on the web site. Luckily for us, this is the site that developers use to upload data and not the live site. It is fully secured with windows authentication and SSL, but I wouldn't recommend this for live, production websites.