<script type="text/javascript">
    $(".ajax__fileupload_dropzone").bind("drop", function () {
            checkfilesize();
    });
    $(".ajax__fileupload_queueContainer").bind("click", function () {       
            checkfilesize();
    });  
    $(".ajax__fileupload_uploadbutton").bind("mouseenter", function () {
            checkfilesize();
    });
    function checkfilesize() {
        var total_filesize_num = 0;
        var myElements = $(".filesize");
        if (myElements.length == 0) {
            $(".ajax__fileupload_uploadbutton").css("visibility", "hidden");
            return;
        }
        for (var i = 0; i < myElements.length; i++) {              
            var filesize = myElements.eq(i).html(); //$(".filesize").html();     
            total_filesize_num = total_filesize_num + filesize_tonum(filesize);
        }
        if (total_filesize_num > 5) {
            $(".ajax__fileupload_uploadbutton").css("visibility", "hidden");
            alert('Maximum file size is 5MB only! Please select another one.');
            return;
        } else {
            $(".ajax__fileupload_uploadbutton").css("visibility", "visible");
        }
    }
    function countsumfilesize() {
        var sumfilesize = 0;
        var myElements = $(".filesize");
        for (var i = 0; i < myElements.length; i++) {
            alert(myElements.eq(i).html());
        }
    }
    function filesize_tonum(filesize) {
        var filesize_num = 0;
        if (filesize.indexOf("kb") > 0) {
            var space = filesize.lastIndexOf(" ");
            filesize_num = parseFloat("0." + filesize.substr(0, filesize.length - space + 1));
        }
        else if (filesize.indexOf("MB") > 0) {
            var space = filesize.lastIndexOf(" ");
            filesize_num = parseFloat(filesize.substr(0, filesize.length - space + 1));
        }    
        return filesize_num;
    }
    </script>
<ajaxToolkit:AjaxFileUpload ID="AjaxFileUploadImage" runat="server" OnClientUploadComplete="uploadComplete" MaximumNumberOfFiles="1" AllowedFileTypes="gif,png,jpg,jpeg" onchange="checkfilesize(); return false;" />