Adding an image icon to two doctypes (.ppt & .doc)

Hi there, I'm running into a really easy problem to solve on this javascript script - but I can't get my head around it, it's just to auto add an image at the end of a link (basically a gif arrow pointing down), I already have it working on one file extension (.ppt), but also want it to work on .doc:

----------------------------

function fNewdownloadicon ()
{
if (!document.getElementsByTagName) return false;
var links = document.getElementsByTagName("a");
for (var eleLink=0; eleLink < links.length; eleLink ++) {
if (links[eleLink].href.indexOf('.ppt') !== -1) {
links[eleLink].onclick =
function() {
window.open(this.href,'resizable,scrollbars');
return false;
}
/* The below section adds the pop-out image */
var img = document.createElement("img");
img.setAttribute("src", "images/download-icon.gif");
img.setAttribute("alt", "(download document)");
links[eleLink].appendChild(img);
/* End of pop-out image code */
}
}
}
addLoadEvent(fNewdownloadicon);

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I just want to add and extra document type to this line:

if (links[eleLink].href.indexOf('.ppt') !== -1) {
---
I thought this would work:

if (links[eleLink].href.indexOf('.ppt', '.doc') !== -1) {
---
but it doesn't... help! Cheers in advance