Australian production had one JPG image which was not recognised as
such. The `content_type` was missing and trying to generate a URL for a
variant raised an error and crashed the page.
Testing for `variable?` includes testing for `attached?` and is more
defensive.
We configured Paperclip to convert images to JPG in some cases but I
omitted that here because we don't need it. If an image is better
represented as PNG or another format then the user should be able to
choose that.
Some specs were also testing the generated URL but the Active Storage
URL doesn't contain a style name anymore and it's not helpful to test
the URL.
If running in developpement (ie. `Webpacker.dev_server.running?`), use webpacker to actually render/serve the CSS files.
Source: https://stackoverflow.com/a/60541688
