Path.GetExtension(string)

Description

Returns the extension component of the specified path string.

The extension returned includes the . character used to separate the extension from the rest of the path.

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