Hi @clarice_larson and @W_Brunette, thanks for your help.
It seems to me that the text type is just another name for the string type - and the xlsx converter changes text types to string types.
Using elementType works great for strings up to about 60,000 characters long. Any larger than this, and I get Internal Server Error (500) when I try to reset the app server.
We require strings around 1,000,000 characters long. Is there any way to achieve this?