SMTP - Broken images in gmail browser application

Configuration
1

HASS version: 0.30.2
Error: none
Problem: broken images in gmail browser application

I have some shell commands set up that automate moving my IP cam and creates snapshots at 3 positions.
This all works flawlessly using curl. The images are created just fine, and have the proper mime-type: image/jpeg.

The mail arrives in my inbox just fine, also containing some sensor information.

But the images are broken. I’ve searched the logs but found no error. The call to the notifer went A-OK:
<Event call_service[L]: service=mailgun, domain=notify, service_call_id=redacted, service_data=message=redacted, title=redacted, data=images=['/home/hass/.homeassistant/redacted', '/home/hass/.homeassistant/redacted', '/home/hass/.homeassistant/redacted']>

I also push the images using telegram, and this works fine. The images also show up in the Gmail Android app. Weirdly enough. So only in the browser it does not work.

My config looks like this:

- name: Mailgun
  platform: smtp
  server: !secret mg_server
  sender: !secret mg_sender
  username: !secret mg_username
  password: !secret mg_password
  recipient: !secret mg_recipient
  port: 587
  starttls: true

And the script:

- service: notify.mailgun
  data:
    title: 'redacted'
    message: 'redacted'
    data:
      images:
        - /home/hass/.homeassistant/redacted
        - /home/hass/.homeassistant/redacted
        - /home/hass/.homeassistant/redacted

Does anyone know how I can fix this?