The default icons for the file types will determine what program is used for opening a particular file when it doesn't have its own icon. For a standard 3.1 installation there are only 5 types of icon: Disk, drawer, trashcan, tool and project. Only the project one is opened by a program, which is defined in the icon information window. Setting the default tool to Multiview will make it attempt to open the file using datatypes, so it will work for any file you have a datatype for.
In 3.1, those default icons are in ROM, so aren't changeable - does anyone know if putting them in ENV:sys/ will override the ROM versions without some add-on?