Get image extention before uploading using javascript

<SCRIPT type="text/javascript">
    function ValidateFileUpload() {
        var fuData = document.getElementById('fileChooser');
        var FileUploadPath = fuData.value;
// var f = fuData.files.item(0); // get only the first file from the list of files
// var filesize = f.size;
// alert("the size of the image is : "+filesize/1024+" bytes");

//To check if user upload any file
        if (FileUploadPath == '') {
            alert("Please upload an image");

        } else {
            var Extension = FileUploadPath.substring(
                    FileUploadPath.lastIndexOf('.') + 1).toLowerCase();

//The file uploaded is an image

if (Extension == "gif" || Extension == "png" || Extension == "bmp"
                    || Extension == "jpeg" || Extension == "jpg") {

// To Display

                if (fuData.files && fuData.files[0]) {
                    var reader = new FileReader();

                    reader.onload = function(e) {
                        $('#blah').attr('src', e.target.result);
                    }

                    reader.readAsDataURL(fuData.files[0]);
                }

            }

//The file upload is NOT an image
else {
                alert("Photo only allows file types of GIF, PNG, JPG, JPEG and BMP. ");
fuData.value="";
            }
        }
    }
</SCRIPT>
<form>
<input type="text" />
<input type="file" name="dataFile" id="fileChooser" onchange="return ValidateFileUpload()" />
<input type="reset" />
</form>