How do I save screenshots onto the computer?

I'm wondering how I'd go about saving a CanvasSnapshot onto the player's computer.

Originally I was following this tutorial on youtube which is exactly what I want to do but seems like it turned into an old method:

Once the image was saved, it appears to be corrupted and unusable by any image viewing program so is there any other method to have this accomplished?

https://www.scirra.com/forum/request-nw-js-write-image-file_p1031519?#p1031519