Function is_binary 

pub fn is_binary(path: &Path, check_bytes: usize) -> bool

Returns true when the file looks like binary content.

The heuristic reads the first check_bytes bytes of the file and checks:

• whether a null byte (\0) is present — classic binary signal
• whether the bytes fail UTF-8 decoding

Any IO error is treated conservatively as “binary” so the file is skipped.