File Download being thrown

Hi there,
I'm pulling my hair out here. I have nginx and php installed they work just fine...as long as I type localhost/index.php into my browser. If I just type localhost it throws a download dialog box.
it's the same with a simple index.html. I have to explicitly state the full path and file name in my browser or else a file download is thrown.
if you know of why this might be happening i'd love to know. thanks
here's my default site config:

server {
listen 80;
root /var/www;
index index.php index.html index.htm;
location ~ /.php$ {
fastcgi_pass unix:/var/run/php5-fpm.sock;
fastcgi_index index.php;
include fastcgi_params;
}
}