Well actualyl it will pick up whatever is first in your server settings, i.e. whatever comes up in your browser. The decision lies with the server. So if you have listed them: index.html, index.htm, index.php, index.exe ... then index.html will be seen by google, otherwise with index.htm, index.html, index.php, index.exe ... index.htm will be sent to google.
Depends on your server settings. Simply check with a browser.
<-- expanding on what killroy posted --> The server can be setup to deliver any page you want. The most common defaults setting are index, default and home. The files can be .html, .htm, .php,.cgi or, anything a browser can understand.