Upload and verify one private chat document

Upload and verify one private chat document. Send multipart/form-data with exactly one file field, or application/json with fileName, optional mediaType and complete file data as canonical padded base64. Do not include a data URL prefix. The shared byte validator chooses the canonical file type. Unknown fields and noncanonical encoding are rejected. Validation and private storage verification finish synchronously; the response is ready for use in ordered attachmentIds on chat start or send. There is no finalize call. Supports PDF, DOCX, UTF-8 text/source/config, Markdown, CSV, JSON and supported images. Maximum 20 MiB of decoded bytes per file, ten pending files and 40 MiB total per person and organization. Pending uploads expire after 24 hours. Upload retries create another ID; recover lost responses with the pending list. Requires agents.write and a user OAuth token. Organization API keys cannot use this operation. Requires rollout-agent-access-writes and allow-public-agent-writes. New upload and message binding also require allow-document-processing.