Imported 3D models are not always converted to OBJ format. The content of ZIP files are not modified for example. But you shouldn't rely on this because it could change one day, the reason why files are suffixed by .pref to avoid users changing them.
If you want to edit a 3D model at OBJ format, you should rather import it then export it with the 3D view > Export to OBJ format menu item.