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("SomeFileAtfoo.extension")); }
using System.IO;
using UnityEngine;
using System.Collections;

public class Example : MonoBehaviour {
    void Start() {
        Debug.Log(Path.GetExtension("SomeFileAtfoo.extension"));
    }
}
import System.IO
import UnityEngine
import System.Collections

public class Example(MonoBehaviour):

	def Start() as void:
		Debug.Log(Path.GetExtension('SomeFileAtfoo.extension'))