There is also the concept of what have been called “sidecar files”. A file with the same name as another, and with related content.
I.e.
Constitution.md
Constitution.txt
Constitution.png
Another possibility is using @LucCogZest’s Hook app to link related files together. There is a great screencast here https://screencastsonline.com/tutorials/mac-apps/hook-for-macos
Finally, an application like Curio allows importing or linking just about anything into its idea spaces, drawing links between, inserting mind maps, lists, photo albums, etc.
Tinderbox and TheBrain are also contenders in this area, as is DEVONthink (Notebooks, Typora, etc.). I suppose it all comes down to what works best for your own mental models.