In each written Insight, use the 'Add attachments' button to share files in your Insight.
Here, you can upload files that you want to share with other participants on the Item or on the Plan. This is best used for static documents that are informative for the team and are unlikely to change often (i.e. Project charters, final agenda, approved product designs, etc.).
These files will be collected in the Shared Files section of the aside for easy access and organization. This allows you to keep these files handy, rather than having to scroll through all of your Insights to find the file in the future.