oEmbed is another form of previewing a URL which requires talking to a provider. By default the media repo has oEmbed previews disabled for privacy concerns, however this can be changed with the following configuration:

  oEmbed: true

The media repo ships with oEmbed's official providers list bundled in and will use that if an alternative providers.json is not specified. The providers.json is considered an "asset" for the media repo, and can be changed by specifying an asset path as a command line flag.