Unity3D Editor Script that renames .atlas files on import to .atlas.txt. Put this script in an "Editor" subfolder.

AtlasFileRenamer.cs

using UnityEngine;
using UnityEditor;
using System.IO;

/// <summary>
/// editor script that renames imported .atlas files to .atlas.txt files
/// </summary>
public class AtlasFileRenamer : AssetPostprocessor
{
    static void OnPostprocessAllAssets(string[] importedAssets, string[] deletedAssets, string[] movedAssets, string[] movedFromAssetPaths)
    {
        bool refresh = false;

        foreach (string importedAssetPath in importedAssets)
        {
            string ext = Path.GetExtension(importedAssetPath);

            // rename .atlas file to .atlas.txt
            if (ext == ".atlas")
            {
                string newFileName = Path.ChangeExtension(importedAssetPath, ".atlas.txt");

                // delete old file if it exists
                AssetDatabase.DeleteAsset(newFileName);

                // rename new file
                FileUtil.MoveFileOrDirectory(importedAssetPath, newFileName);

                // mark editor for refresh after all renamings
                refresh = true;
            }
        }

        if (refresh)
        {
            AssetDatabase.Refresh();
        }
    }
}