A debugging walkthrough for fixing missing MKV video thumbnails on Arch Linux. The root cause was a MIME type mismatch: Thunar sends `video/matroska` to Tumbler via D-Bus, but the ffmpegthumbnailer thumbnailer file only listed `video/x-matroska`. The fix involves creating a custom `~/.local/share/thumbnailers/mkv.thumbnailer` file with the correct MIME type, then restarting tumblerd. The post also covers useful debugging techniques: using `pacman -Ql` to inspect package files, `xdg-mime` and `bash -x` to trace MIME detection, `dbus-monitor` to observe what Tumbler receives, and `gio info` to check GLib-based MIME resolution. A patch to ffmpegthumbnailer was later merged upstream.
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# Symptoms and Debugging# Customizing ffmpegthumbnailer’s Tumbler Entry# Demo Video1.6K Impressions