A response body. Most handlers return ResponseBody::Bytes built from
an in-memory Bytes buffer; the File variant
exists so large disk-backed objects can be streamed straight from the
filesystem to the HTTP body without being materialized into RAM. The file
handle is opened by the service handler while it still holds the
per-bucket read guard, so the reader sees a consistent inode even if a
concurrent PUT/DELETE renames or unlinks the path before dispatch streams
the body.