Function exists_beside

pub fn exists_beside(folder: &Path, kind: Kind) -> bool

Whether an image of this kind is already sitting beside the music, under either extension write_image can produce.

Fanart.tv’s auxiliary artist images are tracked by disk presence, so a pass needs to ask “is there one already” before downloading it. Checks both extensions because the writer picks jpg or png from the bytes it was given, never from what the caller asked for, so a caller checking ahead of the download cannot know which one a previous run wrote.