Add better filters/search for messages and attachments
Have a panel in the sidebar (similar to Slack) to view all available files/your files for the team/channel. Filters by file type and being able to search the filename would also be nice.
Have additional search filters (again similar to Slack) such as the time/duration or the has: qualifier (e.g. has: link)
Charles Chu commented
Instead of another panel, could integrate it into the search function using keyword such as "@files" and show a list of attachments with ability to sort the view.