If you look at the sites IndiaTimes and Rediff you will notice their pages end with tags .cms, and it is a web portal. Our company is interested in making such portals, can any one help and show some light on this regard.
You could use any extension you like. It's a setting in your webserver. I could, for example, tell my webserver to treat .jao files as ASPX files.
Update: There's an extensive list of Content Management Systems available at http://www.cmsfind.com/ and here and some more info here
You can also use http://builtwith.com/default.aspx to get some idea about the technology stack.
Rediff: 80% PHP - no opensource frameworks/CMS used in the main portal. Almost everything is homegrown. They also use PERL, Python, ASP.NET, Java, C and Ruby On Rails.
IndiaTimes: Mostly Java
The extension of the files should be the least of your concerns. I would say the product and features you provide should be something you focus on.
The extension for the file can be set using Apache .htaccess redirect. I think FrogCMS one of the CMS I used for a project, let me choose the file extension.(i.e mask .php to appear as .html), but it did not fit the requirement.
I would suggest looking for a CMS which meets your requirement and then working towards modifying the CMS to enable dynamic extensions.
Indiatimes is NOT java. They strangely use mix of Delphi and Microsoft .net. Some of their channels are on Java with websphere. CMS is their own in-house CMS system using dlls built with delphi.
/ex-developer for indiatimes