You can simply make it by REGEX:
Form:
<form method="post" action="" enctype="multipart/form-data">
<div class="uploadExtensionError" style="display: none">Only PDF allowed!</div>
<input type="file" name="item_file" />
<input type="submit" id='submit' value="submit"/>
</form>
And java script validation:
<script>
$('#submit').click(function(event) {
var val = $('input[type=file]').val().toLowerCase();
var regex = new RegExp("(.*?)\.(pdf|docx|doc)$");
if(!(regex.test(val))) {
$('.uploadExtensionError').show();
event.preventDefault();
}
});
</script>
Cheers!