APNG files dont have any clear way to detect them. For APNg unawware applications they appear as normal PNGs, and they will show the first screen. Detecting them is a bit of a hassle.
APNGs can be detected by seeing if the data contains a acTL block before a IDAT block.
this solution is adapted to python from this response: https://stackoverflow.com/a/4525194/5997749
def is_apng(a: bytes):
acTL = a.find(b"\x61\x63\x54\x4C")
if acTL > 0: # find returns -1 if it cant find anything
iDAT = a.find(b"\x49\x44\x41\x54")
if acTL < iDAT:
return True
return False