After upgrade ffmpegthumbnailer to 2.0.10-1, the Nautilus stop to create thumbnails. If you execute ffmpegthumbnailer manually, you will get the following message: Locale not specified. Check LANG, LC_CTYPE, LC_ALL
The upstream already fix the problem, but Arch Linux package doesn't apply the patch yet. Rebuild the package with the patch or wait Arch Linux to update.