On the fly content type detection.

main.go

// Tee plus LimitWriter.
// LimitWriter taken from: https://github.com/kubernetes/kubernetes/blob/579e0c74c150085b3fac01f6a33b66db96922f93/pkg/kubelet/util/ioutils/ioutils.go#L39-L70
//
package main

import (
	"bytes"
	"flag"
	"io"
	"io/ioutil"
	"log"
	"net/http"
)

// LimitWriter is a copy of the standard library ioutils.LimitReader,
// applied to the writer interface.
// LimitWriter returns a Writer that writes to w
// but stops with EOF after n bytes.
// The underlying implementation is a *LimitedWriter.
func LimitWriter(w io.Writer, n int64) io.Writer { return &LimitedWriter{w, n} }

// A LimitedWriter writes to W but limits the amount of
// data returned to just N bytes. Each call to Write
// updates N to reflect the new amount remaining.
// Write returns EOF when N <= 0 or when the underlying W returns EOF.
type LimitedWriter struct {
	W io.Writer // underlying writer
	N int64     // max bytes remaining
}

func (l *LimitedWriter) Write(p []byte) (n int, err error) {
	if l.N <= 0 {
		return 0, nil
	}
	if int64(len(p)) > l.N {
		p = p[0:l.N]

	}
	n, err = l.W.Write(p)
	l.N -= int64(n)
	return
}

var (
	link = flag.String("l", "https://upload.wikimedia.org/wikipedia/commons/transcoded/4/4d/Wikipedia_Edit_2014.webm/Wikipedia_Edit_2014.webm.480p.vp9.webm", "link to check")
)

func main() {
	flag.Parse()
	log.Printf("fetching %v", *link)
	resp, err := http.Get(*link)
	if err != nil {
		log.Fatal(err)
	}
	defer resp.Body.Close()
	var (
		buf bytes.Buffer
		tee = io.TeeReader(resp.Body, LimitWriter(&buf, 512))
	)
	n, err := io.Copy(ioutil.Discard, tee)
	if err != nil {
		log.Fatal(err)
	}
	log.Printf("read: %d", n)
	log.Printf("detected content-type: %v", http.DetectContentType(buf.Bytes()))
	// 2020/05/15 23:43:32 fetching https://upload.wikimedia.org/wikipedia/commons/transcoded/4/4d/Wikipedia_Edit_2014.webm/Wikipedia_Edit_2014.webm.480p.vp9.webm
	// 2020/05/15 23:44:18 read: 13174381
	// 2020/05/15 23:44:18 detected content-type: video/webm

}