model_and_admin.py

class Release(models.Model):
    ... # a lot of fields
    tarball = models.FileField(blank=True)
    wheel = models.FileField(blank=True)
    checksum = models.FileField(blank=True)


class ReleaseAdmin(admin.ModelAdmin):
    # What I want is for the fields tarball, wheel and checksum to be optional
    # on the model, but required in the admin form

    def get_form(self, request, obj=None, **kwargs):
        form_class = super().get_form(request, obj=obj, **kwargs)

        class ReleaseModelForm(form_class):
            def __init__(self, *args, **kwargs):
                super().__init__(*args, **kwargs)
                for artefact_field in ["tarball", "wheel", "checksum"]:
                    self.fields[artefact_field].required = True

        return ReleaseModelForm