The firewall will record that the file was served but take no actions and make no recommendations.
It is entirely plausible for a DOS attack to focus on resources and not on pages and actions where you have the possibility to interact with the request and response.
You may have files that you'd expect the web service not to serve, however a misconfiguration may leave your size exposed and the firewall detects such attempts and will understand that the user has malicious intentions. If disabling protection of static files this discovery will not take place and the firewall will be less effective
Target Platforms: Windows 7, Windows Vista SP1 or later, Windows XP SP3, Windows Server 2008 (Server Core not supported), Windows Server 2008 R2 (Server Core supported with SP1 or later), Windows Server 2003 SP2