multiWriter.go

// Source:
// Go Playground: https://play.golang.org/p/KjBCcudT1JF

package main

import (
	"bytes"
	"crypto/md5"
	"encoding/hex"
	"io"
	"net/http"
	"os"
)

func main() {
	var f *os.File

	buf := new(bytes.Buffer)
	hash := md5.New()

	io.Copy(io.MultiWriter(buf, hash), f)

	req, _ := http.NewRequest("PUT", "https://example.com", buf)
	req.Header.Set("Content-Md5", hex.EncodeToString(hash.Sum(nil)))

}