[jQuery] Passing a string via variable results in NaN

I've got a little problem with passing two strings $FilePath and
$FileName to the PHP file and I've got no clue why. When I do it this
way everything's Ok

ppost : {
                'Path' : 'temp/',
                'File' : 'Myfile.jpg'
            },

Of cause I need to do this via variable but when I do it in this way:

 $FilePath ="temp/";
 $FileName ="Myfile.jpg";

ppost : {
                'Path' : [+$FilePath],
                'File' : [+$FileName]
            },

Everything that my PHP code recives is NaN for both variables. Could
you tell me why and what would be the proper way?

The entire script:

        function ajaxFileUpload()
        {
                $("#loading")
                .ajaxStart(function(){
                        $(this).show();
                })
                .ajaxComplete(function(){
                        $(this).hide();
                });
 $FilePath ="temp/";
 $FileName ="Myfile.jpg";

                $.ajaxFileUpload
                (
                        {
                                url:'doajaxfileupload.php',
                                secureuri:false,
                                fileElementId:'fileToUpload',
                                ppost : {
                                'Path' : [+$FilePath],
                                'File' : [+$FileName]
            },
                                dataType: 'json',
                                success: function (data, status)
                                {

                                },
                                error: function (data, status, e)
                                {
                                        alert(e);
                                }
                        }
                )

                return false;

        }