HELP solving biplobi challenge

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I have been trying to understand this challenge.
But not getting anything. I tried some XXE nothing seems ok.
Please do me some help.

https://biplobi.com/ctf/view/24

[Exploit-Baba]

I have also tried XXE

[Shouvo]

me too

[error]

Someone has solved the challenge.

[Exploit-Baba]