Browser node should download files only on request

[mantycore]

When new post is created, its files should be replicated only by server nodes.

• Author node broadcasts the post object
• Server nodes, upon receiving the post, request the attached files from the author (this can be potentially toilsome for the author)
• Alternatively, author node sends the attachments to the server nodes known by it, and they send them to more server nodes?
• Browser nodes only display placeholders for the attachments when the post is first received
• When browser node user clicks on the placeholder, the node requests the file (from the known server nodes?)
• Maybe create and broadcast a small preview for images?

[mantycore]

Implemented the plan A (without the broadcast, for now). Previews are not implemented yet, shod create a separate task.