There is only one file type that uses the .MSPX extension:
Microsoft ASP.NET Web Page
Web page developed using Microsoft's presentation framework for ASP.NET; processed by a Microsoft Internet Information Services (IIS) Web server; dynamically generates content that is passed to the client's Web browser.
MSPX Web pages are similar to .ASPX pages, but use ".mspx" as an alternative extension.
Updated: May 6, 2009