//Remember to import IO to make Path work.
import System.IO;

function Start () {
    //This will print ".extension".
    Debug.Log(Path.GetExtension("/Some/File/At/foo.extension"));
}

using System.IO;
using UnityEngine;
using System.Collections;

public class example : MonoBehaviour {
    void Start() {
        Debug.Log(Path.GetExtension("/Some/File/At/foo.extension"));
    }
}

import System.IO
import UnityEngine
import System.Collections

class example(MonoBehaviour):

    def Start():
        Debug.Log(Path.GetExtension('/Some/File/At/foo.extension'))