Microsoft didn’t even mention this change, but it can be seen from the beta version that the current Explorer will only display the folder icon set by the system default. Although the new File Explorer is better to use, the thumbnail function has been quietly removed. However, in Windows 11, Microsoft has improved the File Explorer. If the folder contains pictures, videos, documents, and even code files, they will also be displayed. The thumbnail here refers to the displayed content of the folder.
In Windows 10, File Explorer will display thumbnails of built-in files in the folder, so you can easily find the files contained in it.
