Hey!
I’m working on a save system where the files written to disk are meant to be edited/moved/selected from Explorer by the end users. Because I’m using my own file extension, is it possible for me to set my game up so on the end user’s computer the custom created files are using my own icon, rather than the default text icon?
Yes, you need an installer to install the game on the target computer. Or some sort of script which changes the icon.
You can find a ton of free installers on the market nowadays like Inno Setup for example. They also allow you to write small scripts to do these kind of work.
As of how to change a file system icon is way out of the boundaries of Unity so I won’t answer them, I will let you google the solution for yourself.