Django admin decorator to create a confirmation form action, like the default delete action works

admin.py

from .models import Post, Category
from .decorators import action_form


class PostCategoryForm(forms.Form):
    title = 'Update category for the selected posts'
    myfile = forms.FileField()
    category = forms.ModelChoiceField(queryset=Category.objects.all())


@admin.register(Post)
class PostAdmin(admin.ModelAdmin):
    actions = ['change_category']
  
    @action_form(PostCategoryForm)
    def change_category(self, request, queryset, form):
        category = form.cleaned_data['category']
        return queryset.update(category=category)

decorators.py

import functools

from django.contrib.admin import helpers
from django.template.response import TemplateResponse


def action_form(form_class=None):
    def decorator(func):
        @functools.wraps(func)
        def wrapper(self, request, queryset):
            form = form_class()

            if 'confirm' in request.POST and request.POST:
                form = form_class(request.POST, request.FILES)
                if form.is_valid():
                    obj_count = func(self, request, queryset, form)
                    self.message_user(request, '%s objects updated' % obj_count)
                    return None

            context = dict(
                self.admin_site.each_context(request),
                title=form_class.title,
                action=func.__name__,
                opts=self.model._meta,
                queryset=queryset, form=form,
                action_checkbox_name=helpers.ACTION_CHECKBOX_NAME)

            return TemplateResponse(request, 'admin/form_action_confirmation.html', context)

        wrapper.short_description = form_class.title

        return wrapper

    return decorator

form_action_confirmation.html

{% extends "admin/base_site.html" %}
{% load i18n l10n admin_urls %}

{% block bodyclass %}{{ block.super }} app-{{ opts.app_label }} model-{{ opts.model_name }} delete-confirmation
  delete-selected-confirmation{% endblock %}

{% block breadcrumbs %}
  <div class="breadcrumbs">
    <a href="{% url 'admin:index' %}">{% trans 'Home' %}</a>
    &rsaquo; <a href="{% url 'admin:app_list' app_label=opts.app_label %}">{{ opts.app_config.verbose_name }}</a>
    &rsaquo; <a href="{% url opts|admin_urlname:'changelist' %}">{{ opts.verbose_name_plural|capfirst }}</a>
    &rsaquo; {{ title }}
  </div>
{% endblock %}

{% block content %}
  <ul style="padding: 0">
    {% for object in queryset.all %}
      <li style="list-style: none; float: left; margin: 5px">
        {{ object }}
      </li>
    {% endfor %}
  </ul>
  <hr>
  <br>
  <form action="" method="post" enctype="multipart/form-data">{% csrf_token %}
    <fieldset class="module aligned">
      {% for obj in queryset.all %}
        <input type="hidden" name="{{ action_checkbox_name }}" value="{{ obj.pk|unlocalize }}"/>
      {% endfor %}
      <div class="form-row">
        {{ form }}
      </div>
    </fieldset>
    <div class="submit-row">
      <input type="hidden" name="action" value="{{ action }}"/>
      <input type="submit" name="confirm" value="{% trans "Confirm" %}"/>
      <a href="#" onclick="window.history.back(); return false;"
         class="button cancel-link">{% trans "No, take me back" %}</a>
    </div>
  </form>
{% endblock %}

I do same your instructions but exist values not autofill in confirmation form, right?