Function is_opaque_payload 

pub fn is_opaque_payload(path: &Path) -> bool

Whether prov can read path as text — a recognized body format (Markdown/Djot/HTML) or a whole-file metadata format (YAML/JSON/…). Its negation is an opaque payload: a file prov treats as bytes (an image, a PDF, a font, any binary) and never parses. An attachment is exactly a whole-file metadata sidecar whose content points at such a payload, which is how an arbitrary file gains workspace-linked metadata without being able to carry frontmatter itself.