Deprecated since 3.11.
https://peps.python.org/pep-0594/#imghdr

typesystem support version <= 3.10

3.11 releases just in one week. The typesystem will support it without any doubt.

Additional dependency lib is not cost-effective. Or do not check the image type.

such as : filetype OR puremagic in pypi

This can be an optional dependency for those, who wants to use File fields.

Maybe Image is unnecessary. Just define the File, right?

It would be nice if File() can validate the file type. So we can specify that we want image, audio, video, etc.
Something like

File(accept=['image/jpeg', 'image/png'])

This should answer your question. But IMO, a dedicated Image() field is a nice utility.