The problem actually is that your custom extensions are not mapped in IIS to Asp.Net ISAPI. Here is what actually happens when you have an aspx request:
Request -> IIS -> Asp.Net ISAPI -> Sitefinity
In other words, IIS redirects all requests for ASPX to the Asp.Net environment, and Sitefinity is able to process the code. When you request a regular file, or html page, IIS processes it (but not Asp.Net framework, since this extension is not mapped to it). If the file is there, then IIS serves it, otherwise the server returns the original 404 page. If you want to map all requests to Asp.Net, you can do it with wild card mappings (it is described in our User Manual) or, you can only the extension you want to be processed by Asp.Net. This is exactly the case, when we were mapping sflb and tmb extensions in our previos versions. Please refer to our Troubleshooting section
where we show how to add some extension to IIS. Use the same approach for all custom extensions you want.
This should do the trick.
the Telerik team
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