I have just a few files in “My files” tab of the left pane plus a handful of “Bookmarks”. Yet, I have to choose which of them are displayed, leaving most of the left pane empty. “Tags” take more space but the most popular ones are on the top anyway. My hypothesis is that this is a common use case and most of the users have no more than a handful of files and several of bookmarks.
My suggestion is to add a new tab to the left pane (named “All”?) that would list all currently available tabs (“My files”, “Bookmarks”, “Tags”) one under another. An alternative would be to use an accordion instead of tabs (like here": https://getbootstrap.com/docs/4.3/components/collapse/#accordion-example but with an option to expand more than one section at one) so the user can decide which tabs should be collapsed and which should be visible.