The issue has been identified and we are working on providing a fix and resuming the publishing service as soon as possible.
We are experiencing a downtime of our publishing services for the Apple App Store. All publishing attempts during this downtime are and will be rejected with the following error response:
ERROR ITMS-6000: "Uploads of Mac and iOS apps are only supported on OS X." at SoftwareAssets/SoftwareAsset(MZItmspSoftwareAssetPackage).
To follow the progress of the incident's resolution, please check its status at status.tererik.com
where you can also subscribe to receive notifications.