I haven't been requested any documents yet, so I'm not talking from experience - but I suppose upload it in the field that you have active. I mean, you don't have any other options, right? There were cases when people have been requested docs and didn't have an active upload field at all, so you getting a LoE field could be a minor mistake on their part, or they really don't care how you submit the docs as long as you do.
I say submit it in the field you have and see if the "additional documents request" field in your myCIC status turns to "provided". And if not, try sending a CSE.