Previews can be generated for ZIP files by scanning the ZIP file and getting the first file which can generate an image. 

You can enable this feature by navigating to Admin -> System -> Settings and clicking on Thumbnail Generation Settings. In the settings page view, set "Should a thumbnail be generated based on the first appropriate file in ZIP files?" to Yes and click 'Save'.

If you have existing zip files and would like to have them generate previews, you can trigger a thumbnail regeneration. All ZIP files which can have a preview generated will then generate one. If you require assistance with a thumbnail regeneration, contact our support team.