Hey @Ben_Pettman - it looks like this limitation was inherited from the Cloud implementation of Retool Storage and isn't yet configurable for on-prem deployments. Unfortunately, this means that you'll need to perform the upload via a REST or S3 resource query, instead.
We're currently scoping out a feature that would allow self-hosted users to set this value via environment variables, but there isn't a specific timeline associated with that work. I'll provide an update here as soon as I can!