Serving files with forms authentication

I have an asp.net 4.0 app using forms authentication hosted on windows web server 2008 r2 using iis 7.5.

The application has a calendar screen that users can enter events into. When an event is entered in the calendar it is also updated to an .ics file hosted on the server.  This allows users to subscribe to the calendar events via their personal calendars such as outlook.  

The .ics files are stored in a folder called Calendars in the root of the app. So the file structure in iis looks like this: DefaultWebSite\Calendars\CompanyGuid\calendar.ics

I want to secure those folders so only users that have access to the system can access their company calendar ics file.

However, users will be accessing this file via their outlook calendar and not going through the app, therefore will not be authenticated.

How can I serve these .ics files to users that have a valid userid and password through a program like outlook?

Thanks.