Attach a file to the client brief

Attaches a file to the client brief — the information and documents you hand the client, shown at the top of the portal before the requested items (see client_brief on POST /v1/intakes). Goes through the same pipeline a client's own upload does: real content-type sniffing rather than trusting the declared MIME, HEIC→JPEG conversion, and an antivirus scan before the file is stored — an infected upload is refused outright (422), not stored and hidden. A brief holds at most 10 files; an 11th returns 422. Counts against the account's storage quota. Refused with 409 (reason intake_archived) on an archived intake.