Submit form without uploading image

I am using this pluging to image upload using ajax http://hayageek.com/docs/jquery-upload-file.php

But here in this pluging there isn't any option to make uploader optional with form submit.

Here is my form code i want to make upload field optional.

currently it is not submitting form when i am not uploading any image to uploader.

    <script type="text/javascript">
    //<![CDATA[


    jQuery(document).ready(function() {
    var extraObj = jQuery("#warranty-comment-image-upload").uploadFile({
        url: "<?php echo $this->getUrl('warranty/index/savecomment',array('id'=>$params['id'],'key'=>$params['key']));?>",
        fileName: "img",
        showPreview: true,
        dynamicFormData: function()
        {
            var data = jQuery('#commentForm').serialize();
            return data;
        },
        previewHeight: "128px",
        returnType:'json',
        uploadStr:"<?php echo  $this->__('Upload files') ?>",
        previewWidth: "128px",
        allowedTypes: "jpg,jpeg,gif,png,pdf",
        autoSubmit: false,
        showDelete: true,

    });
    jQuery("#warranty-comment-image-upload-button").click(function() {
         var formToValidate = $('commentForm');
        var validator = new Validation(formToValidate);
        if(validator.validate()) {

            extraObj.startUpload();

        }

    });

});


    //]]>
</script>

Form HTML

<form action="<?php echo $this->getUrl('warranty/index/savecomment',array('id'=>$params['id'],'key'=>$params['key']));?>"  id="commentForm" method="post" enctype="multipart/form-data">

<input type="hidden" name="comment_type" value="<?php echo Mage::helper('warranty')->getCommentType();?>">
<input type="hidden" name="warranty_id" value="<?php echo $this->getRequest()->getParam('id'); ?>">
<div class="fieldset">
    <h2 class="legend"><?php echo $this->__('Warranty Information') ?></h2>

    <?php $warrantyweek = Mage::getStoreConfig('warrantytext/general/warrantytext',Mage::app()->getStore()->getStoreId()); 
         $warrantyMessage = Mage::getStoreConfig('warrantytext/general/warrantymessage',Mage::app()->getStore()->getStoreId()); 
    if($warrantyweek){
    ?>
    <div class="warranty-text"><?php echo $warrantyMessage ?></div>
    <?php } ?>
    <?php if($warrantyExtraMessage = Mage::getStoreConfig('warrantytext/general/warrantyextramessage',Mage::app()->getStore()->getStoreId())): ?>
    <div class="warranty-extra-text"><?php echo $warrantyExtraMessage ?></div>
    <?php endif; ?>

    <ul class="form-list">                                                                 

        <li class="fields">
           <h3 class="tittle"><?php echo $this->__($sectionData['title']) ?></h3>
            <div id="warranty-comment-image-upload" class="extraupload"><?php echo $this->__('Upload Files Here') ?></div>
        </li>
        <li class="wide">
            <label for="comment" class="required"><em>*</em><?php echo $this->__('Comment') ?></label>
            <div class="input-box">
                <textarea name="comment" id="comment" title="<?php echo $this->__('Comment') ?>" class="required-entry input-text" cols="5" rows="3"></textarea>
            </div>
        </li>
    </ul>
</div>
<div class="buttons-set">
    <p class="back-link"><a href="<?php echo $this->escapeUrl($this->getUrl('warranty')) ?>"><small>&laquo; </small><?php echo $this->__('Back') ?></a></p>
    <p class="required"><?php echo $this->__('* Required Fields') ?></p>
    <button id="warranty-comment-image-upload-button" type="button"  title="<?php echo $this->__('Submit') ?>" class="button"><span><span><?php echo $this->__('Submit') ?></span></span></button>
</div>

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