/upload/complete was failing (non-JSON response, missing fileId, or
post-processing timeout) after all bytes were already on the server,
causing upload-manager to retry the entire multi-GB upload — which
corrupts the server-side file since two uploads end up interleaved.
Now /complete failures are swallowed and sessionId is used as the
file_code fallback. Upload is considered done once all chunks are in.
79cf8ad002