Check File Extensions On Multiple Uploads

Script works on the first attachment but not the other two?
<script type="text/javascript" language="JavaScript"><!--
function ExtensionsOkay() {
var extension = new Array();
var fieldvalue = new Array();
fieldvalue[0] = document.customApp.attachment_1.value;
fieldvalue[1] = document.customApp.attachment_2.value;
fieldvalue[2] = document.customApp.attachment_3.value;
extension[0] = ".doc";
extension[1] = ".docx";
extension[2] = ".txt";
extension[3] = ".pdf";

// No other customization needed.
for(var f = 0; f < fieldvalue.length; f++) {
var thisext = fieldvalue[f].substr(fieldvalue[f].lastIndexOf('.'));
for(var i = 0; i < extension.length; i++) {
if(thisext == extension[i]) { return true; }}
alert("Your upload field " + f + " contains an unapproved file name.");
return false;
}}
//--></script>