If a chunked upload causes a warning on the first chunk (e.g. 'exists': File x already exists), the API result does not include an offset.

e.g. when re-uploading https://test.wikipedia.org/w/index.php?title=Special:Log&page=File%3AMP+sounds-pwb-chunked.png

the first chunk has the following JSON API result

{u'filekey': u'12mybcbvc27k.4hakog.25377.png', u'sessionkey': u'12mybcbvc27k.4hakog.25377.png', u'result': u'Warning', u'warnings': {u'exists': u'MP_sounds-pwb-chunked.png'}}

Version: unspecified
Severity: minor