About os.Chmod in SaveUploadedFile

[NezhaFan]

I submitted a PR #4181 , which has been merged. But there's one question that remains.

os.MkdirAll checks whether the directory exists, and if it exists, it does nothing. But os.Chmod does not check whether the folder permissions are the same as currently, it will cause syscall.Chmod to change permissions every time ？

after merged: