If you want to be able to watch/preview the HEIC images, you can edit the available mime type for Nextcloud’s Viewer. (I do this to our company’s NC so customers can use viewer for PDFs and employees can get previews/thumbnails of TIFFs, AIs and PSDs. For HEIC, just go to your NC’s ‘apps/viewer/js/viewer-main.js’ and add ‘image/heic’ and ‘image/heic-sequence’ to the mimetype list (about halfway down, I hgihly recommend using VSCode+a formatter to make finding it easier)