Goで親タイプオブジェクトのサブタイププロパティにアクセスするにはどうすればよいですか?

<code>package main

import (
    "fmt"
    "time"
)

type File interface{}

type Audio struct {
    File
    Duration time.Duration
}

type Image struct {
    File
    Width  uint
    Height uint
}

func main() {
    var files = map[string]File{
        "1": Audio{
            Duration: 14 * time.Second,
        },
        "2": Image{
            Height: 9989,
            Width:  1111,
        },
        "3": Image{
                        Width:  1234,
            Height: 5678,
        },
    }
    for k, v := range files {
        switch v.(type) {
        case Audio:
            fmt.Printf("%s: Audio %d seconds", k, v.Duration / time.Second)
        case Image:
            fmt.Printf("%s: Image %dx%d", k, v.Width, v.Height)
        }
    }
}
</code>