trick

test.go

package main

import (
	"fmt"
	"os"
)

type MyFile struct {
	MyField string
	*os.File
}

func (f *MyFile) Foo() string {
	return f.MyField + f.Name()
}

func main() {
	f, _ := os.Open("./test.go")

	// Initialize embedded type.
	file := MyFile{"foo", f}
	fmt.Printf("%s", file.Foo())
}