One of the files weighs an unusually large 1.79 MB, suggesting that there is something hidden there using steganography.
It was stepic algorithm, which hides text in images by slightly changing their color.
TIP
This example helped me to write a solution script : click
from PIL import Image
import stepic
img = Image.open('upz.png')
flag = stepic.decode(img)
print(flag)picoCTF{fl4g_h45_fl4g51d83cb1}