slack files & url with headers

I'm building a slack bot (python/slack-bolt).Trying to share a private image within the app home tab.

The home tab is genereted using client.views_publish.

{"type": "image","title": {"type": "plain_text","text": "test image",             },"image_url": "https://files.slack.com/files-pri/<private_ref>/test.jpg","alt_text": "test"                    },

Now image_url isn't valid as this is a private file.But the file can be accessed via the requests with headers:e.g. this downloads the file correctly:

curl https://files.slack.com/files-pri/<private_ref>/test.jpg --header "Authorization: Bearer <bot_token>" --output test.jpg

How can the url for an image be constructed for the slack blocks so the private image is available to the bot homepage?