Conditional field for file blueprint: when file is a video then show up

The CMS Questions
1

I have one media.yml blueprint for video, audio, and image files. I want the last field to show up only when a video is uploaded. Is this possible? The conditional field property does not work yet.

title: Media
accept:
  extension: bmp, gif, jpg, jpeg, png, svg, webp, m4v, mov, mp4, ogg, webm, m4a, mp3
fields:
  caption:
    type: writer
    marks:
      - italic
      - underline
      - strike
      - link

  alt:
    type: text
    label: Alternative text
    required: true

  orientation:
    type: toggles
    Label: Orientation
    options:
      - Landscape
      - Portrait
    default: landscape

  preview:
    type: files
    label: Poster
    max: 1
    query: file.siblings
    when:
      accept: video/*
2

The when option only works with field values, so using the accept option will not have any effect.

The only workaround would be a hook that saves the string video to a hidden field when a video is uploaded, and then use this field’s value in the condition.