code: func MIMETypeFromReader(r io.Reader) (mime string, reader io.Reader) {
  	var buf bytes.Buffer
  	_, err := io.Copy(&buf, io.LimitReader(r, 1024))
  	mime = MIMEType(buf.Bytes())
  	if err != nil {
  		return mime, io.MultiReader(&buf, errReader{err})
  	}
  	return mime, io.MultiReader(&buf, r)
  }
docs: // MIMETypeFromReader takes a reader, sniffs the beginning of it,
  // and returns the mime (if sniffed, else "") and a new reader
  // that's the concatenation of the bytes sniffed and the remaining
  // reader.