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...

I encountered this problems while developing a new site using a .less file. This could happen in ASP.Net Development if the file extension is not recognized. To resolve this issue, I had to use a combination of 3 solutions.

You do this by adding a handler to you project's Web.config file and possibly changing the WCF components.

Let's start with the easiest implementation:
1- Open your project's Web.config file
2- Under <system.webServer> add the MIME mapping to handle the file extension you need, like so:

<system.webServer>
    <staticContent>
        <!-- .LESS FILES HANDLER FOR COMPILING IN CSS -->
        <mimeMap fileExtension=".less" mimeType="stylesheet/less" />
        </staticContent>
</system.webServer>"

3- Rebuild the solution and watch for the same error. Most likely this should resolve the issue handling .less files. You can add handlers for any type of file with another <mimeMap fileExtension=" mimeType="" />

If that didn't resolve the problem, you will also need to change your WCF components. Here's what you need to do:
1- Go go Control Panel
2- Click on Turn Windows feature on or off
3- Enable all the components under .NET Framework 3.5 (includes .NET 2.0 and 3.0), which include Windows Communication Foundation HTTP Activation and Window Communication Foundation Non-HTTP Activation.
4- Enable all the components under .NET Framework 4.5 Advanced Services, which include the ASP.NET 4.5 and WCF Services

Hopefully it Windows will load all the additional components after a couple of minutes and you just need to rebuild your solution to work.

If you don't want to go through all the manual changes, than what you can do is to add a new NuGet package to your project solution. These are the steps:
1- Right click the solution
2- Choose "Manage NuGet Packages"
3- Click the "Search Online" and type less
4- Click the applicable solution for installation and start using.

The NuGet package should download all the necessary components. This process should modify your Web.Config file appropriately and install all the dependencies for proper LESS support.