How check multiple (dot) of file name in javascript

String extension = "";
int i = fileName.lastIndexOf('.');
int p = Math.max(fileName.lastIndexOf('/'), fileName.lastIndexOf('\\'));
if (i > p) {
    extension = fileName.substring(i+1);
}

var val = $(this).val().toLowerCase();
var regex = new RegExp("(.*?)\.(pdf)$");
 if(!(regex.test(val))) {
$(this).val('');
alert('Please select only pdf file format');
}

var _validFileExtensions = [".pdf"];    
function Validate(oForm) {
    var arrUpFls = oForm.getElementsByTagName("input");
    for (var i = 0; i < arrUpFls.length; i++) {
        var flUpload = arrUpFls[i];
        if (flUpload.type == "file") {
            var szFileName = flUpload.value;
            if (szFileName.length > 0) {
                var blnValid = false;
                for (var j = 0; j < _validFileExtensions.length; j++) {
                    var sCurExtension = _validFileExtensions[j];
                    if (szFileName.substr(szFileName.length - sCurExtension.length, sCurExtension.length).toLowerCase() == sCurExtension.toLowerCase()) {
                        blnValid = true;
                        break;
                    }
                }
                
                if (!blnValid) {
                    alert("Sorry, " + szFileName + " is invalid, allowed extensions are: " + _validFileExtensions.join(", "));
                    return false;
                }
            }
        }
    }
  
    return true;
}

//call it from fileupload control (Html control)
< form onsubmit="return Validate(this);">
  File: < input type="file" name="my file" /><br />
  < input type="submit" value="Submit" />
< /form>