Modern Python Standard Library Cookbook

How it works...

When the provided filename is a string containing the path of a file, imghdr.what is called directly on it.

This just returns the type of the file or None if it's not supported.

If, instead, a file-like object is provided (a file itself or a io.BytesIO, for example) then it will peak the first 32 bytes of it and detect the header based on those.

Given that most image types have a header with a size in the order of little more than 10 bytes, reading 32 bytes ensures that we should have more than enough to detect any image.

After reading the bytes, it will go back to the beginning of the file, so that any subsequent call is still able to read the file (otherwise, the first 32 bytes would be consumed and lost forever).