The os package makes use of a *File struct rather than an interface. The authors acknowledged this was a mistake but it's some of the oldest code in Go and Go's backwards compatibility guarantee has meant that they cannot fix that.

Since I author a $SHELL written in Go, being able to add in my own *File methods would have allowed me to add in some cool features. But I've found workarounds in most cases. It's just not as clean code as it could have been.