Microsoft has updated the icons used in File Explorer to be the new Fluent icons. Microsoft announced last year that they were beginning a process of migrating the existing Windows 10 icons to their Fluent design system. With a greater aesthetic comes a loss of functionality as the new icons no longer provide a preview of a folder’s contents. Users have complained that the new File Explorer icons do not show previews of the files inside on them. Some of these icons have already made it into the public releases of Windows 10.
Source: https://www.bleepingcomputer.com/news/microsoft/new-windows-10-file-explore-icons-favor-design-over-function/