@arhadthedev do you have time to finish it any time soon? :)

Notice that IANA currently uses the "media types" term:

[RFC2046] specifies that Media Types (formerly known as MIME types) and Media
Subtypes will be assigned and listed by the IANA.

Just an idea, feel free to ignore it: sometimes the following approach may be used to estimate --help message. Try to read man page generated by help2man. Perhaps epilog may be added to ArgumentParser constructor, however I am unsure concerning recommendations within the Python project.

Not sure if it's my configuration or not, but test_guess_type_conflicting_with_mimetypes fails on my openSUSE 15.5. I think the reason is that my /etc/mime.types contains:

image/x-pict                                     pict pict1 pict2 pic pct

Right, from the commits looks like it took a while for @arhadthedev to find a good candidate. Let's update it.

@picnixz Can you find another that's not in your /etc/mime.types?

I'm no more on my Linux session, but I'll do it tomorrow. Can't we mock the array containing the files where everything is looked up?

Yes, that sounds better than having to worry what different systems happen to have.