title: ImageField not validating
body: <p>Why isn't Django's <code>ImageField</code> throwing a validation error here?</p>
  
  <pre class="lang-py prettyprint-override"><code>    # field in model
      image_mobile = ImageField(
          upload_to='static/images/',
          blank=True,
          null=True
      )
  
      # in test
      from django.core.files.uploadedfile import SimpleUploadedFile
  
      attachment = SimpleUploadedFile("file.mp4", b"file_content", content_type="text/plain")
      obj.image_mobile = attachment
      obj.save()
      self.assertEqual(obj.image_mobile, '')
  </code></pre>
  
  <p>Outputs this:</p>
  
  <pre><code>AssertionError: &lt;ImageFieldFile: static/images/file_7wanB5P.mp4&gt; != ''
  </code></pre>
  
  <p>From the documentation:</p>
  
  <blockquote>
    <p>Inherits all attributes and methods from FileField, but also validates that the uploaded object is a valid image.</p>
  </blockquote>