It's a simple challenge with 300 points :)
command "ls" Display files available on the server.
**index.php shell.jpg flag.txt **
Then through the description we pass the command "**history**" and we found
1  ZXh0c2  mFfc23  VjdXJ4  lX2Zp5  bGVfc6  mVhZG7  Vy
Decode base 64 we found (**extra_secure_file_reader)** 
Finally to read the file (**flag.txt)** we just type **extra_secure_file_reader flag.txt** and flag is displayed.
flag is : YUBITSEC{shello_shello_are_you_thereo}