CreateTxt.cs

using System.IO;
using UnityEditor;
using UnityEngine;

// Unity editor script that adds .txt to asset creation list
public class CreateTxt : Editor
{
    private static string filename = "NewTextAsset";

    [MenuItem("Assets/Create/Text/.txt", priority = 1000, secondaryPriority = 10000)]
    private static void CreateNewTextAsset()
    {
        // Getting current directory
        string directory = AssetDatabase.GetAssetPath(Selection.activeObject);
        if (Path.HasExtension(directory))
            directory = Path.GetDirectoryName(directory);

        // Generating filename
        string file = Path.Combine(directory, filename + ".txt");
        if(File.Exists(file))
        {
            for (int i = 1; i < 1000; i++)
            {
                file = Path.Combine(directory, string.Format("{0} {1}.txt", filename, i));
                if (!File.Exists(file))
                    break;
            }
        }

        // Creating file
        using (File.Create(file)) { };

        //Importing created asset
        AssetDatabase.ImportAsset(file);
        TextAsset text = (TextAsset)AssetDatabase.LoadAssetAtPath(file, typeof(TextAsset));
        Selection.activeObject = text;
    }
}