Additional features, settings, or issues not covered by specific Microsoft Teams categories
Found a solution that work in the New Teams:
Problem: Tabs added as Websites pointing at SharePoint links do not display and do not allow correct functionality to users (e.g. download files is not working).
Edit your web address pointing to your SharePoint from this:
https://tenantcompany.sharepoint.com**/:f:/r**/sites/YourSite/Shared%20Documents/folder
to this:
https://tenantcompany.sharepoint.com/sites/YourSite/Shared%20Documents/folder
Add a Tab, choose SharePoint as app, then change to Any Site page and paste the new URL.