I also wound up smack in the middle of a 5 year old original post. Playing off the last comment, I could not change default in the default apps system settings by file type page. There was no option for Cura shown or any choose another app there. However I was able to change it by right clicking an stl file and selecting Open with -> Choose another app, More apps, Look for another app on this pc, Select Ultimaker Cura folder and then choose Cura.exe and click Open. Make sure to check mark Always use this app to open .stl files. Now it appears as default app and an option to pick in the Default Apps system settings, go figure.