file upload on server

function checkFile(obj) {

        var fileElement = document.getElementById(obj);
        var fileExtension = "";
        if (fileElement.value.lastIndexOf(".") > 0) {
            fileExtension = fileElement.value.substring(fileElement.value.lastIndexOf(".") + 1, fileElement.value.length);
        }
        if (fileExtension == "gif" || fileExtension == "GIF" || fileExtension == "JPEG" || fileExtension == "jpeg" || fileExtension == "jpg" || fileExtension == "JPG" || fileExtension == "PNG" || fileExtension == "png") {
            return true;
        }
        else {
            alert("You must select a GIF,JPEG,JPG and PNG file for upload");
            document.getElementById(obj).value = '';
            fileElement.focus();
            return false;
        }
    }