slackblocks-3.1.0: test/spec/fixtures/valid/blocks/image_block_slack_file.json

{
    "type": "image",
    "block_id": "fake_block_id",
    "slack_file": {
        "url": "https://files.slack.com/files-pri/T0123456-F0123ABC456/kitten.png"
    },
    "alt_text": "An incredibly cute kitten."
}