At Admin -> Settings -> Switches, the "Download listing attachments zip" switch enables a download link on the listing details page with which visitors can download all the attached files on that listing as a single ZIP file. If the listing has 30 images, for example, they can get those all at once instead of having to download each image individually. Note that the zip download link will only appear when there are at least two files attached to the listing.

This functionality requires that PHP's zip extension is loaded on the server. The php.ini memory limit will also need to be large to avoid memory exhaustion errors when generating the zip.