javascript - How can I upload files asynchronously?

I would like to upload a file asynchronously with jQuery.

$(document).ready(function () {
    $("#uploadbutton").click(function () {
        var filename = $("#file").val();

        $.ajax({
            type: "POST",
            url: "addFile.do",

            enctype: 'multipart/form-data',
            data: {
                file: filename
            },
            success: function () {
                alert("Data Uploaded: ");
            }
        });
    });
});

File

Instead of the file being uploaded, I am only getting the filename. What can I do to fix this problem?