Media play thumb local image

I have created an automation that will play a mp3 file from the media-source dir on my google nest speaker. Now i want to add a thumbnail as well; so i can see this extra details on my phone (google home app). If i use an external url (some image from the internet) then it works fine; but i can’t get an image file from the media-source working.

my action looks like this:

service: media_player.play_media
data:
  media_content_id: media-source://media_source/local/my_song.mp3
  media_content_type: music
  extra:
    thumb: media-source://media_source/local/my_song.png
    title: My song
target:
  entity_id: |
    {{ trigger.entity_id }}