[Suggestion] Display media type in hidden notification previews, and fix the latter for secret chats

With notification previews hidden, alongside group, channel, contact, or other private chat names, a text saying "Message" is displayed regardless of the contents of a message, which may be a GIF, a sticker, audio, a video, a location, or an image; and when a message from a secret chat gets received, the default "Notification" text placeholder, without the name of a sender, is shown.

Please follow Apple's Human Interface Guidelines:

Based on the user's settings, notification previews may be hidden for privacy. In this case, (...) To provide the user with sufficient context, your app should provide custom text that succinctly describes notification content, like Friend Request, New Comment, Reminder, or Shipment.

The issue identifies hidden notification previews and secret-chat notifications as the entry points, but names no files or tests. Start by tracing where preview placeholder text is selected for ordinary and secret-chat messages, then check handling for GIFs, stickers, audio, video, locations, and images. Done means previews describe the media type and secret-chat notifications include the sender name without exposing message contents.