Thanks for the speedy response. Sadly that didn't fix my issue, so assuming it should work I did some poking around to see what might be the cause of the rejected mime type. TLDR I'm on version 9.0.17 and should try upgrading.
More detailed explanation if anyone finds this topic by searching in the future, it looks like json is being enforced here
https://git.cloudron.io/platform/box/-/blob/v9.0.17/src/server.js?ref_type=tags#L268
but the CLI sends a multipart form, so it can attach the tar bundled app here
https://git.cloudron.io/platform/cloudron-cli/-/blob/master/src/actions.js?ref_type=heads#L699
that's cleared up in the later version 9.1.0
https://git.cloudron.io/platform/box/-/blob/v9.1.0/src/server.js?ref_type=tags#L275
Hopefully that will resolve my issue