code: func WriteFileFromReaderWithModTime(ctx context.Context, bs blobserver.StatReceiver, filename string, modTime time.Time, r io.Reader) (blob.Ref, error) {
  	if strings.Contains(filename, "/") {
  		return blob.Ref{}, fmt.Errorf("schema.WriteFileFromReader: filename %q shouldn't contain a slash", filename)
  	}
  
  	m := NewFileMap(filename)
  	if !modTime.IsZero() {
  		m.SetModTime(modTime)
  	}
  	return WriteFileMap(ctx, bs, m, r)
  }
docs: // WriteFileFromReaderWithModTime creates and uploads a "file" JSON schema
  // composed of chunks of r, also uploading the chunks.  The returned
  // BlobRef is of the JSON file schema blob.
  // Both filename and modTime are optional.