Downloads a stored external file as an attachment by external_files.file_id. The route is skipped by API-key middleware and relies on session/tenant resolution through createTenantKnex and StorageService. Storage lookup is tenant-scoped to external_files.tenant and is_deleted=false, but the handler performs no explicit per-document authorization; any user resolved into the tenant can download a file by fileId. The response body is binary and Content-Type is taken from stored file metadata.