Copyright (c) 2012-2016 Hyperion Entertainment and contributors.
DefIcons manages what pseudo icon is assigned to a file without an icon, you can use DefIcons Prefs to set the default programs used to open different types of file.
AmigaOS may well be a bit hard to understand if you come from the Windows/Mac/Linux world. There are lots of things that can be a little tricky to understand, and one of them is how to get a file such as a JPEG, AVI or even an MP3 file to open with a desired program when double clicking the file's icon in the Workbench. AmigaOS has an installed viewer called MultiView, however this program is somewhat limited and you may want to use a different program to view certain file types.
You could right click on an AVI or MP3 file and then select 'Information...', and from the icon menu browse to your program of choice, e.g. AmigaAMP for an MP3. The problem is that only that single file will then be associated with AmigaAMP. This is where DefIcons comes in. DefIcons allows you to manage and customise the default programs for opening different file types.
On the left side of the window you will see the file types. In this example we are going to edit the program used to open all JPEG files that have a pseudo icon. Scroll down the list to jpeg, found under 'picture' and double click on 'jpeg'. Now a new window has appeared where you can make the necessary changes. Click the 'Icon' tab. Here you can change the file association by removing the current association with MultiView. Click to the button to the right of the text 'Default tool'. You now will now be asked to choose the ability which program you want to associate with the file type. You can choose any program on your computer, be sure to pick one that can load the file type, JPEG in this example. Browse to the program you wish the file type to open in, in this example the third party application 'LoView' is used. Click the 'Save' button and then also click 'Save' on the 'Deficons preferences' window.