Png

A 16-frame APNG hides one flag character per frame in the top-left pixel of each frame’s decompressed scanline data; every standard stego avenue is a dead end, so decompressing all 16 fdAT/IDAT streams and reading byte 4 of each yields the flag.

A ‘PNG’ that file() calls a broken JPEG is a PNG with a forged signature and vandalized chunk names; repairing the header reveals an rMQR (rectangular Micro QR) barcode that ZXing decodes to the flag.