Firefogg returns invalid metadata for files with non-ASCII characters in the name. Found a report upstream that is actually about UploadWizard: https://bugs.launchpad.net/firefogg/+bug/1507203. This is a Firefogg bug, but we could probably work around it.
|Duplicate||matmarex||T136592 Try Firefogg, find out if it's broken and fix it|
|Resolved||matmarex||T136879 Firefogg returns invalid metadata for files with non-ASCII characters in the name|
So… we can't actually work around it. My initial description of "fails explosively" was actually correct. It was just that we were looking at the metadata first and that was also failing, but when I did a workaround in that, it turned out that the actual encode process is also broken.