read a binary file to find its type

[snippsat]

It should be enuff to test for b'\xff\xd8\xff',then it's a jpg.
Start bits:

$JPEG = "\xFF\xD8\xFF"
$GIF  = "GIF"
$PNG  = "\x89\x50\x4e\x47\x0d\x0a\x1a\x0a"
$BMP  = "BM"
$PSD  = "8BPS"
$SWF  = "FWS"

with open('some.gif', 'rb') as fd:
    file_head = fd.read(3)
print(file_head)

Output:
b'GIF'

and 255, 216, 255 in integers

>>> s
b'\xff\xd8\xff'
>>> [i for i in s]
[255, 216, 255]

You see on wiki page that it decode to ÿØÿÛ using ISO 8859-1.

>>> s
b'\xff\xd8\xff'
>>> s.decode('iso-8859-1')
'ÿØÿ'