Ancient thread but I ended up here so others that need help may as well.. In your windows search type "default apps" scroll down to the bottom and select "Choose default apps by file type" (takes a bit to load) look at left column and scroll down until you find the file type you want to set the default program for ("STL" in this case) now on the right column select the existing app icon or the "choose a default" icon a new window will pop up and you can select your desired program from the list.
Cheers