In order to switch your default list view in any folder to display file thumbnails, click on the list/thumbnail switcher on top right for the file list.
You can then see your files displayed with little previews (thumbnails). You can click on the magnifying glass icon on them to open a full preview or hover your mouse on a thumbnail and click on the checkbox that appears to select the files.
When you select one file, checkboxes are displayed on other thumbnails automatically to help you select them. Deselect one-by-one by clicking on the checkboxes again or, press ESC on your keyboard to deselect all / cancel selection.
Thumbnails are displayed for PDF, MS Office, Open Office, TXT files and PNG, JPG and other image formats.
See the video tutorial below: