> then your test should include real input files in it! There's no reason to mock any of this.

That's okay for testing some branches of your code. But not all. I don't want to have to actually crash my hard drive to test that I am properly handling hard drive crashes. Mocking[1] is the easiest way to do that.

[1] For some definition of mock. There is absolutely no agreement found in this space as to what the terms used mean.