My guess is that you propably try to upload a game exported for HTML platform. So a version to be uploaded to a regular website.
For the Arcade you need to export the game using a dedicated export option - Scirra Arcade. When you export and choose the platform, just need to select Scirra Arcade. It's in the first category (Web), same as regular HTML export.
But I agree it would be nice if it's clearly noted on the Scirra Arcade website ;-)