read a binary file to find its type

[DeaD_EyE]

Often it's much more complicated to guess a file type related to the content. Many file formats do have magic number at the beginning. If you want to get the first 3 bytes, then just read the first 3 bytes:

with open('YourFile.ext', 'rb') as fd:
    file_head = fd.read(3)

file_head has now the first 3 bytes. You can make a predefined table for comparison.

If you want just use a library/wrapper for it, you should look for this modules:
file-magic # official
filemagic

More about Magic numbers.

There are different ways to detect if a given sub string (bytes) is in a string.
If you want to detect PNG, you need for example 8 bytes.

magic_numbers = {'png': bytes([0x89, 0x50, 0x4E, 0x47, 0x0D, 0x0A, 0x1A, 0x0A])}
max_read_size = max(len(m) for m in magic_numbers.values()) # get max size of magic numbers of the dict

with open('YourFile.png', 'rb') as fd:
    file_head = fd.read(max_read_size)

if file_head.startswith(magic_numbers['png']):
    print("It's a PNG File")
else:
    print("It's not a png file")

It's just an example how you can do it.