Constant BODY_LIMIT_SLACK_MB 

pub(crate) const BODY_LIMIT_SLACK_MB: usize = 16;

Slack added on top of max_file_size_mb for the raw framework-level body limit — covers multipart boundary/header overhead so the app’s own FileTooLarge check (clean 413) is what rejects an oversized file, not axum’s raw limit (generic 400 MultipartError).