code: func MIMETypeFromReaderAt(ra io.ReaderAt) (mime string) {
  	var buf [1024]byte
  	n, _ := ra.ReadAt(buf[:], 0)
  	return MIMEType(buf[:n])
  }
docs: // MIMETypeFromReaderAt takes a ReaderAt, sniffs the beginning of it,
  // and returns the MIME type if sniffed, else the empty string.