You need to also add it to the message dependencies of the ext.uploadWizard
ResourceLoader module - backend logic can look up messages on the spot,
frontend logic cannot.
Probably something like
$wgExtensionFunctions[] = function () {
$wgResourceModules['ext.uploadWizard']['messages'][] =
'mwe-upwiz-license-c';
};
would work. Although UploadWizard should do this itself if it exposes its
messages to be configurable, so I'd say this is a bug.