code: func (x *Exif) JpegThumbnail() ([]byte, error) {
  	offset, err := x.Get(ThumbJPEGInterchangeFormat)
  	if err != nil {
  		return nil, err
  	}
  	start, err := offset.Int(0)
  	if err != nil {
  		return nil, err
  	}
  
  	length, err := x.Get(ThumbJPEGInterchangeFormatLength)
  	if err != nil {
  		return nil, err
  	}
  	l, err := length.Int(0)
  	if err != nil {
  		return nil, err
  	}
  
  	return x.Raw[start : start+l], nil
  }
docs: // JpegThumbnail returns the jpeg thumbnail if it exists. If it doesn't exist,
  // TagNotPresentError will be returned