A new file will have the default name Sectionxxxx.html, where xxxx is the lowest number not in use. It is not really important to be honest, since renaming is easy. Select the files you want to rename and right-click. Select rename and enter the generic name. The files will be renamed with sequential numbers.
The format is not really important since the declaration is correct. Although I must say that I tend to have everything as xhtml myself.
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