NSCompoundPredicate需要注意个数，如果个数小于2个，不要使用NSCompoundPredicate，返回NSPredicate
var predicateArr = Array<NSPredicate>()
if special_file_types.contains(stringValue) {
let common = NSPredicate(format: "kMDItemFSName LIKE '*\(stringValue)'")
predicateArr.append(common)
// predicateArr.count > 1
return common
// predicateArr.count <= 1
return NSCompoundPredicate(andPredicateWithSubpredicates: predicateArr)
}
