code: func (app *appSec) exifReader() (*bytes.Reader, error) {
  	if len(app.data) < 6 {
  		return nil, errors.New("exif: failed to find exif intro marker")
  	}
  
  	// read/check for exif special mark
  	exif := app.data[:6]
  	if !bytes.Equal(exif, append([]byte("Exif"), 0x00, 0x00)) {
  		return nil, errors.New("exif: failed to find exif intro marker")
  	}
  	return bytes.NewReader(app.data[6:]), nil
  }
docs: // exifReader returns a reader on this appSec with the read cursor advanced to
  // the start of the exif's tiff encoded portion.