You can add links to files in the media library URL via the Custom Links panel of the Customizer but for clients this requires multiple steps including cutting and pasting a URL from the media library (from a box that is much smaller than the URL!) and it's not the best user experience. I am surprised that

1. This isn't already part of WP Core
2. I couldn't find a plugin to handle this.

I have looked into the settings but cannot seem to figure this out.

• $wp_customize->add_section
• $wp_customize->add_setting
• $wp_customize->add_control

As my backend coding is not great I have mocked up a visual.

N.B. Someone else has added it to WordPress ideas here. Votes appreciated.