Can I manually mark files as featurettes?

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Jellyfin uses folder structure as a primary means of organizing media. Anything outside of that *might* work, but is likely to have unintended or unintentional consequences. You might be able to rclone it somewhere on your drive, then symlink it in a proper structure elsewhere on your drive to mimic the proper organization to have Jellyfin pick this up automatically, but that's a really weird kludge that's likely to break at the drop of a hat or if you fart too loud.

You could try editing metadata and make sure that the featurettes/specials have correct metadata information according to TMDB/your source of authority in Jellyfin. Can't promise it'll look correct, but it's worth trying with at least one featurette and rescanning to see if it organizes correctly.

The only other option is to enable the folder view option, but I have no idea what that would look like for this type of setup. Your rclone drive could be a hot mess and it may be worse than dealing with this minor inconvenience.