Problem linking file type to Java program

I just bought my first Apple device ever, a MacBook Air, and am now trying to understand macOS. Now I encountered an issue I just don’t understand. I am running a JAVA based software to track income and expenses. The file type was manually linked to the program which now shows as “default” in terms of finder. The icon is plain white. When I click my saved file it opens the program but not the file itself in the program. I am shown the welcome screen and can now choose to open the last used file. How can I make sure that if I click to open the file not only the corresponding program is opened but also the file itself? The software is called jes eür [https://www.jes-eur.de/](https://www.jes-eur.de/) and the file type: .eux