Support multiple new (file) attachments for CreatePost in Team Messaging REST API

Currently CreatePost only supports existing attachments found in RingCentral.

To POST a file, one must POST to '.../glip/files', with a single file. This automatically creates a Post with a single attachment (the file) and nothing else (no text, mentions, etc).

A User should be able to include text, mentions, etc, and any number of (new) attachments in a single Post - as is already supported by the RingCentral Desktop Application.