Upload files and return UUIDs for each

This endpoint accepts a multipart form with one or more files and returns UUIDs for each. If chat_id is provided, files are associated with that chat. If not provided, files are created as standalone uploads that can be linked to assistants later. An optional outlook_provenance text part carries version 1 OutlookFileProvenance JSON (maximum 64 KiB). It must occur exactly once before any files and applies to every file in the request. Upload files with different origins in separate requests. The original EWS ID query parameter remains independent; a containing email's ID never replaces it.