Picto for generated files

Almost the last question for this project:

The application (osx) has a picto (made with PhotoShop). The app makes its own files with extention .jeexcard , like MS Word makes .docx files. How can I give these own .jeexcards files there own picto?

I found the File Type Sets and included a picto. But the corresponding files on my desktop still show text and there preferred app is somehow set to Dreamweaver.