I have create a varien custom form and want to submit the form using prototype ajax.This form contain four fields

two text field and 
two file field.

But when i submit the data using ajax of prototype js the form did not passed two field and on <form tag i have put enctype="multipart/form-data" but does not works. Code:

<form action="bt" method="post" enctype="multipart/form-data" name="new-art-upload" id="new-art-upload">
<input type="text" name="fname" value=""   class="input-text required-entry"/>
<input type="text" name="fname" value="" class="input-text required-entry"  />
<input type="file" name="fileone"  class="required-entry"   />
<input type="file" name="filetwo" class="required-entry"   />
<button type="submit" title="<?php echo $this->__('Save The Art') ?>"  class="button newAdd_Sub" onclick="newartUpload.submit(this)"><span><span><?php echo $this->__('Save Art') ?></button>
</form>

Script:

<script>
    var newartUpload=new VarienForm('new-art-upload');
    newartUpload.submit=function(button,url){
    if(this.validator.validate) {
        var form=this.form;
        var oldUrl = form.action;
        if (url) {
           form.action = url;
        }
        var e=null;
        try{
          // this.form.submit();
           new Ajax.Request(this.form.action,{
            method:this.form.method,
            parameters:this.form.serialize(),
                    contentType: 'multipart/form-data',

            onSuccess:function(transport){
                var response=transport.responseText.evalJSON(true);
               
            }.bind(this)
            });

          
        }catch(e){
        }
        if(e){
            throw e;
        }
    }
    }.bind(newartUpload)
</script>

I guess that it may content type and mostly content type in form/

Main issue is that files input fields are not sended to ajax request