var reader = new FileReader();

reader.onload = function (event) {
    var buffer = event.target.result;
    var binary = "";
    var bytes = new Uint8Array(buffer);
    var length = bytes.byteLength;
    for (var i = 0; i < length; i++) {
        binary += String.fromCharCode(bytes[i]);
    }
    attachment.Data = btoa(binary);
};
reader.readAsArrayBuffer(file);