code: func (c *Manager) AddLibraryItemFile(ctx context.Context, sessionID string, updateFile UpdateFile) (*UpdateFileInfo, error) {
  	url := internal.URL(c, internal.LibraryItemUpdateSessionFile).WithID(sessionID).WithAction("add")
  	spec := struct {
  		FileSpec UpdateFile `json:"file_spec"`
  	}{updateFile}
  	var res UpdateFileInfo
  	return &res, c.Do(ctx, url.Request(http.MethodPost, spec), &res)
  }
docs: // AddLibraryItemFile adds a file