the older we get the harder the life is. here in OverTheWire it's a bit different. so level 12 is much easier if you've already passed the other 11 levels. (Good for you, by the way! what a journey, huh?!)
have you already read the script? actually, there is nothing new for us. we can get the same from the page: you should upload a .jpg file not bigger than 1KB. let's try it.
before that, do you have burp suite? you're gonna need it. I should confess, at first I hated it. now it's one of my best friends. please, download it and come back...I'll wait. if you have it already, you need to know how it works, right? Here I've found a tutorial for you to be quick, but I've learned it and in like 5 hours with this crazy guy.
I made just a random .txt file and tried to upload it to see what is happening when you don't upload the required .jpeg.
echo file_get_contents( "/etc/natas_webpass/natas13 ");