MCP — support end-to-end attachment upload (not just presign) so clients don't need a separate API key

Summary The built-in MCP server can create an attachment record and return a presigned upload target via create_attachment, but it can't actually store the file. Completing the upload is a separate authenticated POST /api/files.create, which fails with 401 authentication_required unless the client also holds an API token and makes that HTTP call itself. It would be great if the MCP could perform the upload end-to-end using the session it's already authenticated with, so MCP-only clients (Claude, Cursor, etc.) can attach files to documents without provisioning and wiring up a separate API key. Current behavior • create_attachment(name, contentType, size) returns uploadUrl (/api/files.create), the multipart form fields, and the final attachment.url (/api/attachments.redirect?id=…). • Actually storing the bytes requires a multipart POST to files.create. Without an authenticated bearer, that request returns: • Attachment reads are already suppo…