IIS Mime types in web site level

Hello,

There are several links in internet which describes howto add custom file support for Sharepoint document librarys, so that they can be opened directly from Sharepoint. For example like there

But is it possible to add MIME type only for specific Sharepoint web applications (in IIS web site level)? I've tried this, but seems that file is still being downloaded as application/octet-stream...
Why MIME type settings under specific IIS site are being ignored?

Viesturs