def_icons are configured through the
SYS:Prefs/Env-Archive/Sys/ folder.
For example in my 2nd screenshot below you'll see I've configured .mp3's to automatically open in AMPlifier and .mod files to automatically open in DeliTracker2. Most picture formats I default to open in Multiview since I have the
WarpDT's loaded for that. Works great this way for me, and I don't have to assign icons to each individual file just to get them to open in the programs of my choosing. Obviously your mileage may vary.

Edit: If you're having additional problems it may help to post screenshots of what you're seeing and what version of Workbench, etc.
