/// <summary>
 /// Set text of the post. Will override the previous value if it was set.
 /// </summary>
 /// <param name="attachments"></param>
 /// <returns>same instance for a method chaining.</returns>
 public ActivityContent AddMediaAttachments(IEnumerable <MediaAttachment> attachments)
 {
     Attachments.AddAll(attachments);
     return(this);
 }