1:
Code: 
DirectoryIndex index.html
I'm assuming the name of the html file you want default is index.html, if not just change that.

2:
Code: 
RewriteEngine on
RewriteCond %{REQUEST_FILENAME} !-d
RewriteCond %{REQUEST_FILENAME}\.html -f
RewriteRule ^(.*)$ $1.html
change the .html for any file extension you want,
btw that's not my own code, can't remember where I got it from it's just what ive been using for a while