The problem is that IIS will handle the .xml file as a static file and will by default not route the XML file through your MVC application. IIS handles the request and your MVC code never gets a change to route to this file.
There are a few ways around this.
I've found the easiest way to handle this by using the IIS Rewrite module to rewrite the URL from static file URL(s) to an MVC route:
<system.webServer>
<rewrite>
<rules>
<rule name="Live Writer Manifest">
<match url="*.xml"/>
<action type="Rewrite" url="route/xmlfilehandler"/>
</rule>
</rules>
</rewrite>
</system.webServer>
Make sure you have the IIS Rewrite Module installed (separate install from the Platform Installer). If you already are using the Rewrite handler this is the most efficient solution.
As pointed out above by Ben Foster and Jon Galloway's post, you can also map the TransferRequestHandler
at your specific path you want to route. For compactness here's what you need to add to your web.config:
<system.webServer>
<handlers>
<add name="Windows Live Writer Xml File Handler"
path="wlwmanfest.xml"
verb="GET" type="System.Web.Handlers.TransferRequestHandler"
preCondition="integratedMode,runtimeVersionv4.0" responseBufferLimit="0"
/>
</handlers>
</system.webServer>
You can then use an Attribute Route to handle .xml file Urls. For example:
[Route("blog/wlwmanifest.xml")]
public ActionResult LiveWriterManifest() {... }
More info in this blog post:
http://weblog.west-wind.com/posts/2015/Nov/13/Serving-URLs-with-File-Extensions-in-an-ASPNET-MVC-Application