RESOLVED
This was an environmental issue and not a Mautic Problem.
My Mautic instance sits behind an Nginx webserver configured as an internal reverse proxy. This allows me to host websites on many different domains behind a single public IP address.
Nginx was incorrectly configured for my use case as it was set to only allow 50MB client uploads [client_max_body_size].
Probably won’t be useful for most people, but thought I would share anyway.