Upload a file sent in a call's chat (the main room or a breakout): a PDF, Word, PowerPoint, Excel, text or CSV file, or a picture, up to 20 MB.

Stored in the same PRIVATE chat bucket as the pictures and answered the same way: a 60-minute signed URL plus the gs:// ref behind it. Returns {success, url, ref, name, size, content_type}; the client sends a file_message with the URL, the name and the size, and logs it with /chatrooms/{room}/chat/log. Anything else is a plain 400.