>>I guess it doesn't matter
>>
>>It is like .htm and .html
>
>This is different. A MHTML document includes everything necessary to display a Web page into one single file. This is not like a HTML file.


Yes, I know

But the question was what extension to use: mht or mhtml

It doesn't matter - it's like using htm or html

mht is short for mhtml
htm is short for html