I always put ".htm" at the end of the filenames, a legacy from the MSDOS era. ".html" is allowed, but ".htm" works just as well, so why bother?
To delimit folder names (directory names), always use slashes (/), never backslashes (\). Backslashes are a Windows thing, but Internet existed long before Windows was even invented. Most web servers also use Unix (Linux, Free BSD, etc.), and in a Unix system folders are delimited by slashes (/). Browsers written for Windows can also handle slashes quite well.