Difference between .htm and .html in Outlook 365

I'm trying to add a custom HTML signature in Outlook 365. Therefore I create a normal signature and go to ...\AppData\Roaming\Microsoft\Signatures to edit the signaturename.htm file.

But even if I'm defining the language (charset utf-8) it seems to use a different language.

And I just can't copy and paste my code from my .html file to the .htm file. Is there a big difference between .html and .htm?

If I use my .html file in Thunderbird everything works fine.