In the meantime Slack does export images and attachments. Not directly though, but through links embedded in the exported posts / messages.
It would be great if the Mattermost import could retrieve files via these links while importing the Slack export!
Thanks @robert, there’s a few things that need to be improved with Slack import and I think that’s one of them - would you be willing to open a feature request on it so we can get feedback from others and prioritize accordingly?
If someone would like to work on contributing this, please let us know on the feature request!