Function path_file_ext 

pub fn path_file_ext(path: &str) -> Option<&'static str>

Returns the lowercased file extension of path as a &'static str.

• Returns Some(ext) for extensions recognized by language_for_extension (e.g. "rs").
• Returns Some("other") for paths that have an extension but it is not in the known set.
• Returns None for paths with no extension or an empty extension.