For files that do not generate a preview automatically (such as ZIP files or virtual items), you can assign a custom preview.
A custom preview is an image or video that visually represents the item and is displayed instead of a generic icon. This helps you quickly identify the right content without opening every file. 

1. Open the item detail view for the content item.
2. In the Item toolbar, select Change preview.

3. In the Select content dialog, you can use the search and filter options to find a suitable image or video to represent the content item.
4. Click on the image or video you would like to use as a replacement for the generic icon. 
5. Select Select.
6. The selected file is now displayed as the preview for the content item.