[jwatzman]

> The concept of an application 'supporting a (specific) file system' sounds slightly ridiculous to me.

This concept is actually way less ridiculous than you might think, for any application which needs any guarantees about data durability, locking, etc (which includes everything from the obvious ones like postgres to things like Dropbox). I found https://danluu.com/deconstruct-files/ a fascinating read diving into this.