Heh, yeah, I have no idea either!
It seems like (at least on windows) when you run the game .exe, it first opens the ‘game.projectc’ file and tries to use it. You can open other .projectc files with it and it works fine unless that file contains weird or missing information.
I would use this for making better tools with Defold (not an intended purpose, I know). At the moment I want it for my multi-image-viewer, so you can double-click on its save files to start the program and open the file, rather than having to start the program, press ctrl-O, find the file, and open it. It’s just much more convenient. Actually I think I already figured out a hacky solution—I should be able to make save files that are like game.project files, but with my extra save information included.
It would be cool to have this feature for other projects though (where the above method definitely won’t work). For example, what if, with my polygon editor, you could double-click a ‘.convexshape’ file anywhere on your computer and the editor would start up and load the file. That would be nice!
Obviously, Defold isn’t meant for this sort of thing, so this isn’t a feature request, I’m just wondering if it’s possible.