The path admin/content/files is missing in Drupal 8.5
I had the same problem and was because the route that creates the page (view.files.page_1) was not recognized by Drupal. Why? I don't know, but I reinstalled the Views UI and the Views module and the problem was solved.

You can read about this problem in Add Item for Files under Content Menu and this is why we added in the Admin Toolbar module a validation for the route. The file module can be installed and the route can be missed from the site.