short report about using filetypes... as already mentioned you need a def_ icon with the same name predefined
Yes same default name, in my case on AROS One I used individual associations for individual video and audio formats (see screenshot icons), by default Wander associates all video or audio to the same icon, to have an icon for each type of extension you have to create datatype descriptors "by individual format"
then you create a appropriate entry in the list and rename it, if def icon exists it is shown. Below you can define how files are identified. I use "pattern", similar to magellan to identify file by ending.
Basically the procedure and values are always the same as Dopus4, Magellan, Deficons (OS3.9) etc..
In fact, I use the same AROS descriptors even on my OS3.9 systems, AfA OS, Amithlon etc..