This highly depends on the installed plugins in your browser. You need to install a plugin that supports the decoding of those mov files. Usually this is not installed. Typically this is a codec package or comes with some video players.
Please check first to run the mov file directly in the browser without Nextcloud in between. For this usually a drag and drop into an empty browser tab should do the trick. If it downloads, then there is no codec available. If it plays back, please open a ticket in the video player app: GitHub - nextcloud/files_videoplayer: 📼 Old video viewer for Nextcloud
from [FEATURE REQUEST] Add support for .MOV format · Issue #9264 · nextcloud/server · GitHub