Hello,
As of r75906 [1] or one of its related revisions, the sessionkey
returned by action=upload is no longer an integer but a string. This
may affect you if your client library is written in a strongly,
statically typed language such as C or C#.
This revision is part of MediaWiki 1.17, which is tentatively slated
for deployment on Wikimedia in one or two weeks and for release a
month or so after that. Other breaking changes previously announced on
this list and not yet live on Wikimedia will also go live then.
Best regards,
Bryan
[1]
http://www.mediawiki.org/wiki/Special:Code/MediaWiki/75906