Some files are not served off of IIS because they are typically part of the building blocks of the website itself. For ASP.NET these are files like *.cs, *.dll, *.config, *.cshtml, etc.
You can find a list of them tied up in the IIS management setting "Filter requests" here:
But if you need to programmatically access this list, it seems tough to find. Is there a good list of these default extensions?
BTW, the IIS website has info on how to enable / disable these globally here:
http://www.iis.net/configreference/system.webserver/security/requestfiltering/fileextensions
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