converting a div content to png/jpg and downloading it user friendly

See more:

ASP.NET

HTML

jQuery

Canvas

i want to convert a div content to image and download it user friendly.
im using following code for it

$("#btnimage").click(function () {
            html2canvas($("#print"), {
                onrendered: function (canvas) {
                    theCanvas = canvas;         
                    Canvas2Image.saveAsPNG(canvas);

                }
            });
        });

this code work but user have to give extension when downloading it. i need to popup a save box with selected extension .PNG instead of .allfiles(*.*) . I found one solution for php . but i m working in asp.net . Is there any solution for this plz help