Function extract_extras_into

pub fn extract_extras_into(
    source: &Path,
    folder: &Path,
) -> Result<Vec<Extracted>, TagError>

Writes out every picture a file carries that is not the front cover.

Into folder/artwork/, never beside the music: the reason is written on crate::coverart::EXTRAS, and it is not a matter of tidiness — an image next to the tracks becomes the album’s cover as far as the scanner is concerned, and a back sleeve promoted to cover is worse than no cover.

Answers one line per picture, said or refused, because a run over a library wants a count of both and the caller cannot tell them apart afterwards. A picture already written is a refusal like any other: nothing overwrites.