Testing file uploads in Flask

The issue ended up not being that when one adds content_type="multipart/form-data" to the post method it expect all values in data to either be files or strings. There were integers in my data dict which I realised thanks to this comment.

So the end solution ended up looking like this:

def test_edit_logo(self):
    """Test can upload logo."""
    data = {'name': 'this is a name', 'age': 12}
    data = {key: str(value) for key, value in data.items()}
    data['file'] = (io.BytesIO(b"abcdef"), 'test.jpg')
    self.login()
    response = self.client.post(
        url_for('adverts.save'), data=data, follow_redirects=True,
        content_type="multipart/form-data"
    )
    self.assertIn(b'Your item has been saved.', response.data)
    advert = Item.query.get(1)
    self.assertIsNotNone(item.logo)