Document addition of validate_image_file_extension() as a backwards incompatible change for ImageField

[rm_]

The following test code snippet stopped working after upgrading django to 1.11 from 1.10.x. The code now raises a validation error because the posted image has not a valid file extension:

class Test(TestCase):
    def test_post(self):
        img = BytesIO(IMAGE)
        data = {
            'image': img,
        }
        response = self.client.post(url, data=data)

The view takes the data and fill the following ModelForm that fails validation:

class Image(models.Model):
    image = models.ImageField(_(u'Immagine'), upload_to='image')


class ImageForm(forms.ModelForm):
    class Meta:
        model = Image

As a workaround, adding a name attribute to the BytesIO object make the code work again:

        img.name = 'ciao.jpg'

[Tim Graham]

We could document the backwards incompatible change. Did you have anything else in mind?

[rm_]

Replying to Tim Graham:

We could document the backwards incompatible change. Did you have anything else in mind?

I think a mention of ImageFields name attribute requirements should be added in the paragraph that suggests to use StringIO / BytesIO here:
​https://docs.djangoproject.com/en/1.11/topics/testing/tools/#django.test.Client.post

[rm_]

PR here ​https://github.com/django/django/pull/8466

[Tim Graham <timograham@…>]

In bdf192c5:

Fixed #28148 -- Doc'd ImageField name validation concerns with the test client.

[Tim Graham <timograham@…>]

In 643413f6:

[1.11.x] Fixed #28148 -- Doc'd ImageField name validation concerns with the test client.

Backport of bdf192c59357a0d8117f6f34c94fb32a51e7a774 from master