How do I change densandstein in my minecraft tp?

I've just run a tp project with paint.net and wondering how to change the sandstone so I changed it shon and when I pulled it into the tp folder with the same name as the default sandstone but it was not as usual "there's a file alredy egsistig with the same name do you want to replace it?" but it was just 2 files with the same name in the folder dan, I deleted the file from the defoult sandstone and have my minecraft open

but the sandstone was displayed as defult what can i do?

Did you use the correct file extension?

Which file ending?