I think you are confused, probably on OS3 + Scalos it works for you because you use Deficons, delete this file and its preferences and you will see that ScalOS will no longer recognize the files.
Look at my the Screenshot, on my AIAB AfA OS, ScalOS works perfectly and Scalos Filetype also works fine, and all files are associated with the right icon and the right tool.
On AROS this you have to have in order to associate files !