FileMaker choose the icon that comes along with the file. If it can not identify the icon it shows like in your operation system a standard icon. If you want to control the icon yourself I would create a separate container field where I saved Container icon based on GetContainerAttribute ( YourContainerField ; "filename" ) and out of that just take out the extension with Right ( value ; number of extension letters ) and have different icons saved in a Settings/Global table.
rather late (i was ill) but nevertheless: thanx Johan. Well, i think i didn't really understand your answer, but as i wrote in german, i think it wasn't quite clear what was my question.