Incompatible with incoming webhook payloads in slack block kit format

[Dalabad]

Summary

Incoming webhook messages in the "new" slack block kit format are ignoring the block content

• https://api.slack.com/block-kit
• https://app.slack.com/block-kit-builder/

Steps to reproduce

• Create an incoming webhook link
• Send the following Webhook content to the webhook url

curl -X POST \ -H 'Content-type: application/json; charset=utf-8' \ --data '{"attachments":[{"blocks":[{"type":"section","text":{"type":"mrkdwn","text":"Some content for the first block"}},{"type":"divider"},{"type":"section","text":{"type":"mrkdwn","text":"Another block"}},{"type":"actions","elements":[{"type":"button","text":{"type":"plain_text","text":"Button 1","emoji":true},"value":"click_me_123"}]}]}]}' \ https://mattermost-webhook-link.com/hooks/............

• This works with all the examples/templates from the "block kit builder" page

Expected behavior

Here is a screenshot of what the webhook should look like on slack.

Observed behavior (that appears unintentional)

This screenshot shows, how the webhook is displayed by mattermost. The attachment part is displayed (visible by the box with the blue line on the left) but the content of the attachment block is completely missing. Blocks can also appear outside of the attachments block as it can be seen in the official block kit documentation.

[amyblais]

@Dalabad Would you be open to confirming your Mattermost server version?

[Dalabad]

@amyblais Sure thing. My testing instance has the following version:

Mattermost Version: 5.21.0
Database Schema Version: 5.21.0
Database: postgres

[amyblais]

Opened a ticket here: https://mattermost.atlassian.net/browse/MM-26729.

[thinkmassive]

I created a very rudimentary solution using mitmproxy:
https://github.com/thinkmassive/mattermost-webhook-slack-proxy

It works for my purposes, but I'll be glad to improve it if anyone has feedback. I'm especially interested to know what FOSS applications people are using that send webhooks in "block kit" format, so I can understand what message formats should be handled until Mattermost has official support.