As far as I know there’s no standardized framework or API to program an extension for. You’d need to hack your way into (modify code that is not ment to be modified) the server if you need something that’s not included with the standard builds.
After 3 years of using seafile and looking for alternatives I can say that it sits between something like git-annex in terms of sophistication and nextcloud or wonkcloud in ease of use (the code for everything else that is not sync related seems to be reviewed and tested by an army of 9yr olds but that’s a different story).
Bottom line:
Use seafile if you want the best and most reliable synching algorithm out there to manage your files. If you need extras consider using a separate server for hosting PLEX and give it access to your files using seadrive, with a reasonable amount of cache.