Function get_extension 

pub fn get_extension(filename: &str) -> Option<&str>

Extract the file extension from a filename, lowercased for comparison.

Returns the extension without the leading dot, or None if there is no extension. The returned slice borrows from filename.

Examples (conceptual, no_std)

• "readme.txt" -> Some("txt")
• "Makefile" -> None
• "archive.tar.gz" -> Some("gz")
• ".hidden" -> None (dot-files with no further extension)