UnifiProtect: Person detect notification event_thumbnail not working for me

I am new to HA and trying to send a notification containing a thumbnail of UnifiProtect Object detection event, but failing miserably.

When I include
{{ state_attr(sensor.<name>_detected_object, 'event_thumbnail')}}
in the message, I see a relative URL starting with /api/ufp/thumbnail/ but when I add it after image, or include it in the attachment data, I get a

Failed to load attachment
URLSessionTask failed with error: unsupported URL

the YAML is

alias: testing
description: ''
  - platform: state
    entity_id: sensor.<name>_detected_object
    to: person
condition: []
  - service: notify.mobile_app_<name>
      title: Testing 1-2-3
      message: Body {{ state_attr('sensor.<name>_detected_object','event_thumbnail')}}
        image: sensor.<name>_detected_object.event_thumbnail
          url: sensor.<name>_detected_object.event_thumbnail
          content-type: jpeg
          hide-thumbnail: false
mode: single

Some pointers in the right direction would be highly appreciated !

Answered here: Custom Component: Unifi Protect - #767 by AngellusMortis