With a Cart plugin enabled, whenever the users are downloading the photos, they're getting the info about possible malicious software included - it happens in Chrome.
Do you have a link? This sounds like either a browser extension, or for some reason Google Chrome believe the images served on your domain are malicious. This does not happen in the demo, and I have never seen it myself. https://demo.files.gallery/?galleries/cityscape
Very little Files gallery can do with this. If Chrome reports this, there must be a reason ...