Multiple file upload docs

[Guillaume LEBRETON]

In the docs, ​https://docs.djangoproject.com/en/dev/topics/http/file-uploads/#uploading-multiple-files
there is this code snippet:

def post(self, request, *args, **kwargs):
        form_class = self.get_form_class()
        form = self.get_form(form_class)
        files = request.FILES.getlist('file_field')
        if form.is_valid():
            for f in files:
                ...  # Do something with each file.
            return self.form_valid(form)
        else:
            return self.form_invalid(form)

I think it's a bit confusing to use form validation and then get the file list directly from the request.
I found that replacing files = request.FILES.getlist('file_field') by files = form.files.getlist('file_field') seems to be more consistent with the usual django form approach.

The best approach would be to access file_field from cleaned_data but it's not really possible with multple files for now.

PR link: ​https://github.com/django/django/pull/16265