If you try to use imagetransfer.py to copy a file from Commons to Wikisource, it will fail with API error copyuploadbaddomain
This is because although the target wiki has URL uploads on, the URL in question isn't OK. However, if it had URL uploading disabled entirely (or the user didn't have the rights to use it), it would happily download and re-upload.
ignore_warnings = self.ignore_warning is True or self._handle_warnings if '://' in file_url and not site.has_right('upload_by_url'): try: file_url = self.read_file_content(file_url) except FatalServerError: return None
This should probably fall back to the download/re-upload method if the upload-by-URL fail due to copyuploadbaddomain.
See also T294824