'Legacy banner' issue

[Ovidiu Suciu]

We have this issue https://github.com/canonical-web-and-design/snapcraft.io/issues/2423 an there is an error message coming from the api.
The name match on "banner.[png|jpeg]" is too aggressive and should only be matched against if the file is of type "screenshot".
Currently is seems to be matching against type "banner".

[Jonathan Hartley (tartley)]

Suspect code:

```
dashboard:/src/devportal/forms.py:2004
# XXX cprov 2019-03-14: quick-and-dirty support for the
# GnomeSoftware (legacy) banner & banner-icon as specially
# named screenshots. Once clients get proper support for
# banner, this 'if' and the helper method can be
# removed along with the corresponding tests.
if not self._is_legacy_banner(uploaded_file.name, image):
```

_is_legacy_banner tests for LEGACY_BANNER_NAMES = ('banner.png', 'banner.jpg')

[Hasan Ammar (hasanammar)]

The fix for this bug has been deployed in production. Legacy/gnome-software banners are not supported for new uploads anymore.