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.

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

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