How to keep file extension (.txt) when renaming using AssetDatabase?

I'm trying to make an editor script to create text files. At the moment I just copy a template .txt file and rename it. The problem is that I WANT the .txt extension to remain after I've created the copy, but it doesn't. In the project folder it just appears as "filename" with no extension, even when I explicitly rename the file with ".txt" appended. It just ignores it completely.

The only reason it bothers me is that I want to be able to double-click the asset in the project folder and have it automatically open in monodevelop, but that only happens if it has the .txt extension. If it doesn't, it opens in TextEdit (I'm on a Mac). If I rename the asset in the project window manually, it sticks, but that's a pain.

Here's my basic code when creating a new asset:

 TextAsset newText = (TextAsset) Resources.Load ("Text/_BLANKFILE", typeof (TextAsset));
 string newFilePath = _createAssetRootPath + fileName ;
 AssetDatabase.CopyAsset (AssetDatabase.GetAssetPath (newText), newFilePath);
 AssetDatabase.Refresh (ImportAssetOptions.Default);

Again, even including an explicit ".txt" in the path name variables doesn't do it. Anyone know how to solve this? Or am I just missing something?