I dunno if you knew about it or not, but I just found the answer myself and thought I'll share with you guys as you help me a lot here on this forum. I just reset standard apps preferences by "reset all Windows recommended apps preferences" in settings app, then went to a .cfg file on my hard disk (in this case a CS GO config file, preferences > open width standard notepad++ , opened any .cfg file and... eureka :) The icon of .cfg extension is back again as well. I'm happy, though I need to associate another new file types too.
It just happened while I was messing with Chrome > Gmail mailto: preferences.