Extensionless URLs for Generic Content based modules is supported. When you enable the Extensionless URLs for your project, you will be able to access your items/pages with or without extensions. If you want Sitefinity to generate the links without extensions, you should change the approriate module provider's urlRewriteFormat. For example, the Generic Content urlRewriteFormat should look like this:
| <clear />
| <add name="Generic_Content"
As you can see, the .aspx extension is removed. Check the attached screenshot in order to see this in action.
the Telerik team