Golang怎样获取文件的mine type

package main

import (
    "os"
    "fmt"
    "net/http"
)

func main() {

    // Open File
    f, err := os.Open("test.pdf")
    if err != nil {
        panic(err)
    }
    defer f.Close()

    // Get the content
    contentType, err := GetFileContentType(f)
    if err != nil {
        panic(err)
    }

    fmt.Println("Content Type: " + contentType)
}

func GetFileContentType(out *os.File) (string, error) {

    // Only the first 512 bytes are used to sniff the content type.
    buffer := make([]byte, 512)

    _, err := out.Read(buffer)
    if err != nil {
        return "", err
    }

    // Use the net/http package's handy DectectContentType function. Always returns a valid 
    // content-type by returning "application/octet-stream" if no others seemed to match.
    contentType := http.DetectContentType(buffer)

    return contentType, nil
}