Upload a file from a published app (public, append-only)

Accepts one multipart file in the field file, plus an empty hidden _honeypot field. The file lands as a document in a library the app owner controls; the app can NEVER read it back, so do not build a gallery of uploads. The response carries a fileRef to echo as _files on a subsequent form submission to link the two. ALWAYS returns 200: on any drop the fileRef is a decoy, so it cannot be used to detect whether the upload was stored.