Can't preview photos in Dropbox folder when files are online-only

In general, online-only files don't contain any data on them, so there shouldn't be any previews for files that are online-only

Err... this is not quite correct. At least, assuming some interpretations of "preview".

Thumbnails, which are the mini versions of images you see when browsing in Finder, should show up (modulo bugs).

Usually we do try to provide thumbnails for online-only files - however, there is a known issue on macOS where for (IIRC) users on macOS 14+ (IIRC), but NOT on our File Provider implementation, we're not able to provide thumbnails for online-only files.

Actual previews (e.g. when using the space bar to trigger an expanded view of an image from Finder), are a bit different.

In some ways, those have the opposite problem. For File Provider, using the space bar shortcut will trigger the file to be downloaded in order to show that high-resolution "Preview". Prior to File Provider, and on macOS < 14 (again, IIRC), I believe the way we integrated thumbnails for online only files also provided low-resolution (~256x256) "previews" when you hit space bar on an online-only file, instead of causing the file to be downloaded.

ptorgus, supposing what you mean by "preview" is what I'm calling "thumbnail", I suspect your team member is not using our File Provider implementation, and that's why they're not able to see thumbnails.

If they're eligible, they should be able to go to (dropbox tray menu) -> Preferences -> Sync, and then follow the prompts under "Dropbox folder updates". You can see https://help.dropbox.com/installs/dropbox-for-macos-support for more information.