Angularjs how to get filename extension?

Hi I am developing angularjs application. I am doing file upload module. I am trying to validate the files. I want to upload only xlsx or xls files. I have developed directive and factory method to upload files as below.

myapp.directive('fileModel', ['fileUploadExcel', function (fileUploadExcel) {
    return {
        restrict: 'A',
        link: function (scope, element, attrs) {
            element.bind("change", function (evt) {
                fileUploadExcel.pendingFiles[attrs.fileModel] = evt.target.files[0];
            });
        }
    };
}]);

Below is my controller.

 $scope.upload = function (filename) {
 fileUploadExcel.doUpload().success(function (success) {
 }).error(function (error) {
});

Below is my factory code.

myapp.factory('fileUploadExcel', ['$http', '$cookieStore', 'cfg', function ($http, $cookieStore, cfg) {
    var LoginID = $cookieStore.get("LoginID");
    var baseurl = cfg.Baseurl;
    var fileuploadurl = baseurl + 'api/Customer/BulkUploadVehicle/';

    var service = {
        uploadUrl: fileuploadurl,
        pendingFiles: {},
        doUpload: doUpload
    };
    return service;
    function doUpload() {
        var filename;
        var files = new FormData();
        files.append('LoginID', LoginID);
        angular.forEach(this.pendingFiles, function (value, index) {
            filename = value.name;
            files.append(index, value);
        });
        var extn = filename.split(".").pop();
        if (extn == 'xlsx' || extn == 'xls') {
            return $http.post(this.uploadUrl, files, {
                transformRequest: angular.identity,
                headers: {
                    'Content-Type': undefined
                }
            })
        }
        else
        {
            return false;
        } 
    }
}]);

I am able to get extn successfully. When my files are of type excel my code works fine. Whenever i upload other than excel i am getting error. I have wrote return false. This is not working here. I should return something else may be. May i know what i should write in else case? Can anyone help me out here? Any help would be appreciated. Thank you.