Steps leading to the error:
- Moved an user page to https://commons.wikimedia.org/wiki/Campaign:bpbpe
- Visited https://commons.wikimedia.org/wiki/Special:UploadWizard?campaign=bpbpe
UploadWizard campaigns are JSON documents, but an user had the idea to create it in a sandbox, then ask for a move... providing to the extension an unexpected content type.
An error to move would be nice, automatic conversion would be nicer.