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Sorry to hear you’re facing problems 
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Nextcloud version (eg, 20.0.5): 27
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The issue you are facing:
If I setup Nextcloud, with only “external storage”, which will be local storage really, but the same folder that Syncthing is syncing to/from, how do I ensure filenames/paths will be indexed, metadata read and all images (if possible videos too) thumbnails are being generated after the files have been locally created?
To ensure smooth web browsing experience, I want to generate-in-advance as much as possible.
I already understand Syncthing and Nextcloud need to use the same user etc. That is no problem for me.
This is really about triggering Nextcloud, for example, by built in mechanism monitoring folders (inotify) or more efficiently, by leveraging the Syncthing API: