func handleExifUpload(ctx context.Context, r io.Reader, filename string, imageType exif.FileType) (io.ReadCloser, error) {
	tmpfile, err := ioutil.TempFile("", "exifremove")
	if err != nil {
		return nil, err
	}
	go func() {
		<-ctx.Done()
		tmpfile.Close()
	}()
	if err := os.Remove(tmpfile.Name()); err != nil {
		return nil, err
	}

	_, err = io.Copy(tmpfile, r)
	if err != nil {
		return nil, err
	}

	tmpfile.Seek(0, io.SeekStart)
	isValidType := false
	switch imageType {
	case exif.TypeJPEG:
		isValidType = isJPEG(tmpfile)
	case exif.TypeTIFF:
		isValidType = isTIFF(tmpfile)
	}

	tmpfile.Seek(0, io.SeekStart)
	if !isValidType {
		log.WithContextFields(ctx, log.Fields{
			"filename":  filename,
			"imageType": imageType,
		}).Print("invalid content type, not running exiftool")

		return tmpfile, nil
	}

	log.WithContextFields(ctx, log.Fields{
		"filename": filename,
	}).Print("running exiftool to remove any metadata")

	cleaner, err := exif.NewCleaner(ctx, tmpfile)
	if err != nil {
		return nil, err
	}

	return cleaner, nil
}
…
func isJPEG(r io.Reader) bool {
	// Only the first 512 bytes are used to sniff the content type.
	buf, err := ioutil.ReadAll(io.LimitReader(r, 512))
	if err != nil {
		return false
	}

	return http.DetectContentType(buf) == "image/jpeg"
}

Warm-up