Reduce structural dependency in contrib.admin 'has_file_field' checks

[btknu]

This ticket is about a very small cleanup. It's possible that a Trac ticket isn't necessary for this, I decided to make one to stay on the safe side.

The PR ​https://github.com/django/django/pull/9298 for ticket https://code.djangoproject.com/ticket/28585 adds code:

            'has_file_field': context['adminform'].form.is_multipart() or any(
                admin_formset.formset.form().is_multipart()
                for admin_formset in context['inline_admin_formsets']
            ),

This snippet invokes is_multipart() on formset's form() object.
But the formset (specifically: BaseFormSet) already provides an is_multipart() method.

It feels cleaner to rely on a single place to calculate is_multipart for a formset.
This way if is_multipart logic gets more complex, we only need to update it in one place and there's less risk of bugs.

I suggest replacing:

admin_formset.formset.form().is_multipart()

with:

admin_formset.formset.is_multipart()

[Tim Graham <timograham@…>]

In 7c3a8b9:

Fixed #30243 -- Simplified ModelAdmin.render_change_form()'s has_file_field.