Filepicker inside a list

I am trying to have a list auf items, one should be a an image. So I trying to achieve that with the filepicker. The problem is, whenever I hit the save button, the chosen image not chosen any more, so its basically not saved.

Outside the list, I can very well save the filepicker.

Anyone knows what I am doing wrong?

here is the exerpt from the blueprint:

YAMLCopy

        header.videos:
          name: videos
          type: list
          style: vertical
          label: Videos
          fields:
            .title:
              type: title
              label: Titel
            .url:
              type: text
              label: URL
            .show:
              type: toggle
              label: Zeigen?
              highlight: 1
              default: 1
              options:
                  1: 'Ja'
                  0: 'Nein'
              validate:
                  type: bool
            .header.a_file:
              type: filepicker
              folder: '@self'
              preview_images: true
              label: Select a file