[javascript] Allow only pdf, doc, docx format for file upload?

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!