Well, this bcs of your system does not know abt your files.
You should just specify handlers for that in registry:
- in here you can put the information abt your extension and how it should be handled (including opening displaying in shell and so on)
Actually I think the main property which you require to add/change is
the default value is the guid of property handler (you can look at it for example for MS office files)
Note: if you specify office handler guid then property may not be displayed in case if MS Office not installed.
Information related to shell extensions: Registering Shell Extension Handlers