Tried to backup/restore from one Confluence system to another. The file is downloaded as zip package, it can be opened, everything looks reasonable. For the restore though I get the following error: "unable to recognise uploaded file type: multipart/form-data", even when it's uploaded to the very same system the backed was created and downloaded from.
What is wrong?

(Adaptavist: There is a Firefox fix for the issue in this comment)