Overwrite a file warning when saving an image with node.js

I have an app written in javascript that lets a user save an image to their disk. This involves node.js. If I ask to overwrite a previously saved image, I get mixed results when it comes to getting a warning from the OS that I am about to overwrite the file. If I attempt to overwrite the image by selecting the old image from the save file dialog, it prompts a warning. If I overwrite an image by simple typing in the same filename, it overwrites the image but does not give any warning.

The node.js function:

var fs = require('fs');
exports.buildFile = function(name, value) {
    if (name.search(/\.[a-z]+/) == -1) {
        name = name + ".png";
    }
    var img = new Buffer(value, encoding='base64');
    fs.writeFile(name, img, function(err) {
        if(err) {
            console.log(err);
        } else {
            console.log("The file was saved!");
        }
    });
};

Just checks to see if the user specified a file type, and if not, appends .png and writes to disk.

Front end:

var global_imageToSave = null;
function screen_shot(fn) {
    global_currModel.menuShouldHide = true;
    global_currModel.setTextDeletables(false,true);

    html2canvas($('body'), {
        onrendered: function(canvas) {
        global_imageToSave = canvas.toDataURL("image/png").split(',')[1];
        global_currModel.menuShouldHide = false;
        global_currModel.draw();
        sc.buildFile(fn, global_imageToSave);
      }
    });
}

Uses html2canvas to take a screen shot and then passes that data and the file name recieved from the html dialog.

HTML Dialog onchange event:

$('[id^="model_sales_asset"]').live("pagecreate", function() {
    $(this).find('.fileDialog').change(function() {screen_shot($('.fileDialog').val());});
});

As you can see the filetype is being appended before any call to the OS to save the file so I can't figure out why it matches filenames only when it is entered automatically by selecting a file.

This behavior is consistent on windows 7 and 8 as well as mac os 10.7.