Follow-up from "Resolve "Improve Web IDE launch performance""

The following discussion from gitlab-ce!25700 should be addressed:

• @pslaughter started a discussion:

  note: A lot of this function came from the original implementation in files_decorator_worker. I cleaned some things up, but was afraid to change too much of it. If we see some refactoring improvements maybe we can create a follow up issue for them 🤷

What to do?

Let's try to reduce the cognitive complexity of the decorateFiles function. See the linked MR for details.