Preview function does not work in a file located in second level

It’s related to the encode/decode I think.

For example:

Files under <https://github.com/HITSZ-OpenAuto/COMP1011/tree/main/slides/22%E7%A7%8B%E8%AF%BE%E4%BB%B6> cannot be previewed on <https://hoa.moe> or <https://prev.hoa.moe>
