Hi @ngcnelson, I think the /collapse behaviour in Slack collapses all images/videos/files, and leaves the text for site previews, is that correct?
In Mattermost /collapse currently works for images and videos from a web link. We would need to look into making it work for site previews and integration posts containing images, but I think that would make sense to do.
For uploaded files, we need to figure out how this should work though since Mattermost allows multiple files to be uploaded per post (so would we only have one collapse icon that collapses all files on the post? where would the icon go? etc). If you or anyone else from the community is interested in writing a spec for how this should work please feel free to come join us in the UX Design channel on https://pre-release.mattermost.com to discuss!
And finally, we don't have any near future plans to make /collapse only collapse previous files in the channel, but if someone from the community would like to work on a pull request for it that would be great. I've created a ticket here on the issue: https://mattermost.atlassian.net/browse/PLT-4380