We don't enforce a lot of the things like this yet because underneath it's just a plain text field at the moment. Same with the API, probably.
Eventually, if/when we allow custom asset types, it'll actually be a number underneath that is a foreign key to the asset_types table, meaning we'd actually have to care what you input or risk a database error.
But yeah, I'd suggest you don't do this long term as nothing is set up to accommodate for it anywhere.
