hexdump -C -n 16 issue-29512.jpeg
00000000  ff d8 ff e0 00 10 4a 46  49 46 00 01 01 00 00 01  |......JFIF......|

ackage main

import (
	"encoding/hex"
	"fmt"
	"image"
	"log"
	"os"

	_ "image/jpeg"
	"image/png"
)

func hexdump(filename string, n int) error {
	f, err := os.Open(filename)
	if err != nil {
		return err
	}
	defer f.Close()

	data := make([]byte, n)
	_, err = f.Read(data)
	if err != nil {
		return err
	}

	fmt.Println(hex.Dump(data))

	return nil
}

func decodeConfig(filename string) error {
	f, err := os.Open(filename)
	if err != nil {
		return err
	}
	defer f.Close()

	config, format, err := image.DecodeConfig(f)
	if err != nil {
		return err
	}

	fmt.Println("Width:", config.Width, "Height:", config.Height, "Format:", format)

	return nil
}

func convert(filename, out string) error {
	f, err := os.Open(filename)
	if err != nil {
		return err
	}
	defer f.Close()

	img, _, err := image.Decode(f)
	if err != nil {
		return nil
	}

	g, err := os.Create(out)
	if err != nil {
		return err
	}
	defer g.Close()
	if err := png.Encode(g, img); err != nil {
		return err
	}
	return nil
}

func main() {
	filename := os.Args[1]

	if err := hexdump(filename, 16); err != nil {
		log.Fatal("hexdump:", err)
	}

	if err := decodeConfig(filename); err != nil {
		log.Fatal("decodeConfig", err)
	}

	convert(filename, "out.png")
}