In Notepad, when saving a new file, click on Save As...

Then in the popup "Save As" window you need to click on the "Save As Type" dropdown and select "All Files" - you should then see all the files in the folder, regardless of type. Then in the filename box, you need to type blah.html, then click on the Save button. This should save the file with name blah.html, which is what I assume you want? (Of course, replace 'blah' above by the actual filename you want to use).

Try it - does that work?