As part of your build process, you want to post artifacts to Slack for easier access for your team. Although it is not possible to send a file via the Slack Step directly, you CAN send a link pointing to a file that can be downloaded from Slack.
Add the Send a Slack Message Step to your Workflow
Generate an artifact as an HTML file.
In the Step, find the Text of the Message to Send input.
Add the HTML link: you can add it either directly to the input or create an Environment Variable with the link as its value and use the variable in the input.
Depending on any privacy settings you have set up for the link, your users can access it via Slack.