Identify a file thanks to its extension - Create a file extension

[Overnight]

From control panel, choose "Folders Options" and "File Types" tab.
Now you can see a list of all file extensions that appear in Windows' registry. For each one, you can identify the usual related program and modify some parameters.
With "New" button, you can create a custom file type with custom parameters and custom icon.

If you can't find the extension you're looking for, try this link : Extensions