Microsoft IIS does not support RSS files by default. Here is a step by step guid eto enabling it.

If you’re impatient, already know your way around IIS and just want the MIME type details, they are:
Extension: .rss
MIME type: application/rss+xml

Otherwise, if you need more information, read on…..

First, open up IIS. It’s under control panel > administrative tools > Internet Information Services (IIS) Manager.

If you want to allow RSS files for all web servers, right click “Web Sites” and click properties. If just for a sincle website, right click the individual website (on the left hand side) and click properties.

You then need to click the [HTTP Headers] tab (image above) and then the [MIME Types] button.

Click New and enter:
Extension: .rss
MIME type: application/rss+xml

Once that’s all done, keep clicking OK until you’re back to the main IIS management console and you’re done.

Test it and it should work.