unix standard is html
windows standard is htm (to fit in with the dos/windows three letter extension naming standard)
It doesn't really matter - either one will work as the directory index (if set that way in the apache httpd.conf which it is here). The only note is that there is an order specified for the directory index so an index.html file (if present) will be seen first and thus will override index.htm if you uploaded both.
The order in which apache looks for a directory index (as specified at http://www.aota.net/HTML/homepage.php4
index.shtml (means the server will process for ssi instead of just serving)
index.shtm (means the server will process for ssi instead of just serving)
index.php (means the server will process as php instead of just serving)
index.php4 (means the server will process as php instead of just serving)
index.php3 (means the server will process as php instead of just serving)
index.cgi (means the server will process as cgi instead of just serving)
index.fquest (the default coming soon page - thus when the client uploads index.htm or index.html that will override even if they forget to delete index.fquest)
This can also be useful in other ways. For example if you install a forum which uses index.php as it's directory index when someone visits yourdomain.com/forum and you want to take it offline while working on it, you can simply upload an index.html file with text such as "please come back in a few minutes - work in progress" and that will be read instead of index.php.