Fix CLI --content option

[bradlc]

This PR fixes the CLI --content option. Following resolution the content config looks like this, so it's actually content.files that needs updating:

{
  files: [],
  extract: {
    DEFAULT: undefined
  },
  transform: {}
}

Side note: is extract.DEFAULT supposed to be undefined here? 🤔 See #5778

[RobinMalfait]

Ah that's a good catch! Also thanks for adding a test!