HTTP Error 404.3 - Not Found
The page you are requesting cannot be served because of the extension configuration. If the page is a script, add a handler. If the file should be downloaded, add a MIME map.
I got this error when trying to let user download .dwg files in IIS 7.5 but a MIME map was not created. Any uncommon files will get you this error. Fortunately, it’s very easy to do.
Open IIS and navigate to your site. Now click on MIME Type (shown below).
Right click and select Add (or click Add… in the right-side menu). You are now presented with a dialog requesting you to enter your filename extension and the MIME type (list of possible MIME types). This will enable you to associate a file extension with a type of content.