Javascript: Validate file extension when uploading files

/*check file extension*/
function CheckFileExtension(obj){
val=obj.value.substring(obj.value.length-4,obj.value.length);
  if (val!='.jpg'&&val!='.gif'&&val!='.bmp'&&val!='.png'&&val!='.jpeg'&&val!='.JPG'&&val!='.GIF'&&val!='.BMP'&&val!='.PNG'&&val!='.JPEG'){
   return false;
  }
}