is it possible to create a droplet that will be launched when I double click onto my own file types like “File.droptxt” and “File.droprtf”. I also want new Icons for those file types. I know, that I can edit which file extensions a droplet accepts, but how to tell that my droplet will be the default app for opening my files?
If you add an extension in Script Debugger’s Resources panel, you can subsequently edit the app’s Info.plist file to change its CFBundleTypeRole from Viewer to Editor. That might be enough. Be careful, though, that no other app uses the same extension.
For icons, it’s considerably more complicated, and probably not something you’d want to attempt with a standard applet. This should give you some idea of what’s involved: