Unexpected files on a memory card will of course not harm a camera. What harm could they do?

Cameras write files in a predefined structure – see for example Design rule for Camera File system – but otherwise the camera behaves pretty much like a computer (which, arguably, it is), i.e. it writes the files it needs to in the available space on the memory card, and ignores everything else.

Cameras have the ability to format memory cards – obviously if you perform this action, you can expect to lose your files.

The main thing though is that you shouldn't use memory cards as general storage. They are not designed for this, and you just run the risk of losing data; it's not worth it. If you value your data more than your storage devices (and you should), then just dedicate a memory card to a camera, and use it exclusively for one purpose. If you need more cards/storage, buy more – it is not expensive.