AssetDatabase.GetCachedIconを使用してアイコンとラベルを表示する。 AssetDatabase.GetAllAssetPaths()で取得するのでファイル数が多いと重くなるかも。

FileIconTest.cs

using UnityEngine;
using UnityEditor;
using System.Collections;
using System.Collections.Generic;

public class FileIconTest : EditorWindow {

    Vector2 scrollPosition;
	
	[MenuItem ("Window/File Icon")]
	public static void Init () {
		FileIconTest.GetWindow(false, "FileIcon");
	}
	

	void OnGUI () {

		scrollPosition = GUILayout.BeginScrollView(scrollPosition, false, false);

		foreach (string path in AssetDatabase.GetAllAssetPaths())
		{
			Texture icon = AssetDatabase.GetCachedIcon(path);
			if (icon != null)
			{
				GUILayout.BeginHorizontal();
				GUILayout.Label(icon, GUILayout.Width(20));
				GUILayout.Label(path);
				GUILayout.EndHorizontal();
			}
		}

		GUILayout.EndScrollView();
	}
	
}