How to convert a document to a .str and a .ttml file

I have a document saved in notepad, but for some reason after saving it, it is appears in properties at .str.txt and .ttml.txt instead of just .str or .ttml, which is what I would like it to be.

I think that I tired it all, following tutorials and I'd be glad to know what I am doing wrong, or simply - not doing?