oobe_help_opt_in_details.htm

HTML page Reference

An .htm file contains a web page in HTML. The extension is the three-letter form of .html, from systems with short extensions. A web browser reads the file and shows the page. The content is plain text with markup tags. Local documentation and help pages frequently use this extension.

A file extension is only a part of the file name. It does not give a guarantee about the format or the content of a file. But on Windows, the extension selects the program that opens the file.