how to bind ajaxStart/ajaxStop to a form?

How to bind ajaxStart/ajaxStop to a form?

I need a loading block to show while uploading the file

Now the form is not submitted at all

var iframe = $('<iframe name="iframe_file_upload" src="'+src_file_upload+'" style="display:none"></iframe>');
 var btn_send = $('<input type="button" />')
     .click(function(){
         $('#form_upload_file').bind('ajaxStart', function(){
             $(this).submit();
         });
         iframe.bind('ajaxStop', function(){
             $(this).load(function(){
                 ...
 

update

var btn_send = $('<input class="btn btn_margin_left" type="button" value="'+Lang.get('BTN_UPLOAD')+'" />')
     .click(function(){
         $('#form_upload_file').ajaxForm(function(response){
             if(response.error){
                 // alert error
             }
             else{
                 // success
             }
 

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