This challenge is [steghide](http://steghide.sourceforge.net/) challenge.
There are many steghide decoder. I used [Steganographic Decoder](https://futureboy.us/stegano/decinput.html).Select challenge picture file input **text/plain**.
And decode you can get Flag.

Original writeup (https://312k.github.io/312k-blog/post/post6/brixelctf-312k-eng/#rufus-the-vampire-cat).