as far as i know there is no possibility to get individual icons for your documents. Here’s a simple tip how I use the document name to make it fast visible what “kind” or “type” of document i use. I just use some special characters as prefix of the name.
E.g. use a ♯ for a scene, a for a character description, a for a setting description etc.
This is cool if you are using references, you can easily see what kind of links you have.
(sorry for my english and greetings from germany)