This checkbox is used to modify the way you see files on your computer: as icons that you can customize by using FileMarker.NET, or as thumbnail previews.
Thumbnail is a small picture, demonstrating the content of a file. Instead the icon of the file, you see the image that tells you what is inside the file without opening it.
Icon view vs thumbnail view in File Explorer for the same file. You see the nature photo on thumbnail even without opening the file.
You will see such thumbnails for graphical and video files only if you use Extra large, Large, Medium, Tiles and Content views while displaying your computer files.
At Small icons, List, and Details view you will see icons of the files in 16x16 size in any case.
Thumbnails are convenient way to work with your files. But technically, they are shown above the file icon. And this is the problem.
FileMarker.NET is used to customize icons. But the icons are hidden under thumbnails, when they are shown.
This picture clearly explains in what cases you will see the icon and when the thumbnail.
You have two options how you can deal with thumbnails and FileMarker.NET:
1. Leave “Always show icons and never thumbnails” option unchecked and work with customized icons only at Small icons, List, and Details view with icons of 16x16 size.
We chose the red icon for the file. In the Details view we see the red icon now. But we still see the thumbnail in the Large view.
2. Turn on “Always show icons and never thumbnails” option and see customized icons at all sizes and in all types of view.
This how your file with nature will display in Details and Large view after setting the checkbox.
Important! Setting this checkbox you will switch off the displaying of thumbnails not for the certain file but for all files of your computer system.
What option will work for you better depends on how often you use views with large icons and what is more important to you: see thumbnails or see customized icons. You can always change your mind and return all the thumbnails the same way, so try and choose what way is more convenient for you.