RewriteEngine On
RewriteCond %{http_host} ^example.com [NC]
RewriteRule ^(.*)$ https://www.example.com/$1 [L,R=301]

RewriteCond %{HTTPS} off
RewriteRule (.*) https://%{HTTP_HOST}%{REQUEST_URI} [L,R=301]

#1)externally redirect "/file.htm" to "/file"
RewriteCond %{THE_REQUEST} /([^.]+)\.htm [NC]
RewriteRule ^ /%1 [NC,L,R=301]
#2)Internally map "/file" back to "/file.htm"
RewriteCond %{REQUEST_FILENAME}.htm -f
RewriteRule ^(.*?)/?$ /$1.htm [NC,L]