code: func (c *Client) FileHasContents(ctx context.Context, f, wholeRef blob.Ref) bool {
  	if err := c.condDiscovery(); err != nil {
  		return false
  	}
  	if c.downloadHelper == "" {
  		return false
  	}
  	req := c.newRequest(ctx, "HEAD", c.downloadHelper+f.String()+"/?verifycontents="+wholeRef.String())
  	res, err := c.expect2XX(req)
  	if err != nil {
  		log.Printf("download helper HEAD error: %v", err)
  		return false
  	}
  	defer res.Body.Close()
  	return res.Header.Get("X-Camli-Contents") == wholeRef.String()
  }
docs: // FileHasContents returns true iff f refers to a "file" or "bytes" schema blob,
  // the server is configured with a "download helper", and the server responds
  // that all chunks of 'f' are available and match the digest of wholeRef.