Wikimedia Bugzilla is closed!

In order to test chunked uploading in production, please enable it as an opt-in user preference.

$wgUploadWizardConfig['enableChunked']='opt-in';

in the UploadWizard section of CommonSettings.php should do the trick. We've set up a page at http://commons.wikimedia.org/wiki/Commons:Chunked_uploads which we'll use to coordinate testing.