Slack posting by webhooks does not support upload of local images

[instantiator]

Context

The SlackWebhook network uses Slack incoming webhooks, which do not support image upload.

This means that images can be included by url (and these will be fetched by Slack) but that local images cannot be included in messages sent to Slack.

Investigation notes

There are a couple of options:

1. Wait until we implement a real Slack client
2. Upload images to an alternate location, and then address them by URL

Option 1 - implement a real Slack client

This is the ideal option. A fully implemented Slack client that uses the API, rather than just a simple webhook, would be able to upload images and is also extensible for future capabilities (ie. notifications, search channels, reply to messages, etc.)

Option 2 - upload to an alternate location

This is a good transition option. We could integrate with a variety of different solutions for this.

• Microbin - an OSS file upload service that can provide short term storage
• Any other social network - we could use other posts made to alternate networks as a file storage solution
• Other solutions... TBC

if we uploaded to another service first (eg. BlueSky) then it may be possible to refer to the post there and to retrieve the images from that post by URL. (This is untested and needs a bit of investigation.)