Django: ImageField in unittests

models.py

class Post(models.Model):
    image = models.ImageField(
        max_length=255,
        upload_to='uploads/blog',
    )

tests.py

from django.test import TestCase
from django.core.urlresolvers import reverse

from blog.models import Post

from StringIO import StringIO
from PIL import Image

class BlogPostTestCase(TestCase):

    def test_admin(self):
        file_obj = StringIO()
        image = Image.new('RGBA', size=(50, 50))
        image.save(file_obj, 'png')
        file_obj.name = 'test.png'
        file_obj.seek(0)

        self.client.post(
            reverse('admin:post_add'),
            {
                'title': 'xxx',
                'image': file_obj,
            }
        )