Why does the Preview panel show the file as an icon instead of its thumbnail image?
In the Preview panel and the Thumbnail panel at the Inspector bar, you can see a thumbnail image which is stored in your catalog database. To import a thumbnail image into a catalog, please turn on the "Generate Thumbnail Images" option in the Scan preferences panel before scanning a disk.
In the Preview panel, you can also see a preview image of the original file when the disc is online.
When the app can not find the original file and the imported thumbnail image, the app will show you a plain icon in the Preview panel.