Posting images with a structured message to Slack

Slack API supports posting a message, uploading a file, but it’s not straightforward to post images with structured message.

First, you can post a message with chat.postMessage, and build a structured message using attachments or blocks. But it doesn’t allow you to post files. The best you can do is to use Image Block and pass a URL to an image. But, this means that these images are publicly available. Also, you’ll see these links in the message.

You can also include a link to an image in a text and let Slack unfurl it, but again, the image needs to be public.

Second, you can upload a file with files.upload. You can post it to a channel with channel, but you can only post it with a simple message using initial_comment.

To post a file with a structured message, you need to upload a file with files.upload, and update it with chat.update. When you update an existing message, you can specify attachments and blocks. It even allows you to specify file_ids to add more files uploaded with files.upload.

It looks weird that chat.postMessage doesn’t support file_ids, and files.upload doesn’t support attachments and blocks, but you can rely on chat.update.