Sometimes (maybe more with larger files) UploadWizard fails after the "Describe" stage with "Internal error: Server failed to publish temporary file" like in the screenshot below:

AIUI at this point the image has been successfully uploaded (into a temporary location?) and thumbnailed (see the thumb in the screenshot above), but then something goes wrong later in the process.

AFAICT swift and thumbor are both currently behaving themselves, so it'd be useful to know what UploadWizard is doing at this point, and if it could be made to produce and/or log a more helpful error message to aid debugging.

[this task arises from T353498 which was about failure to complete the initial upload ("Upload" step), but that seems to be working]