If you application pool is running in Integrated mode then you can do the following.
Add the following to your top level web.config.
<system.webServer>
<modules>
<add name="FormsAuthenticationModule" type="System.Web.Security.FormsAuthenticationModule" />
<remove name="UrlAuthorization" />
<add name="UrlAuthorization" type="System.Web.Security.UrlAuthorizationModule" />
<remove name="DefaultAuthentication" />
<add name="DefaultAuthentication" type="System.Web.Security.DefaultAuthenticationModule" />
</modules>
</system.webServer>
Now you can use the standard ASP.NET permissions in your web.config to force forms authentication for all files in the directory.
<system.web>
<authorization>
<deny users="?" />
</authorization>
<authentication mode="Forms" />
</system.web>