How do I configure Webform and CiviCRM for event registration including optional donation
I understand what you’re trying to do.

Try set it to Donation. That will make your Contribution level financial type = Donation - but looking at the lineItems within that you should see two lineItems: 1) Financial Type associated with the CiviCRM Event Page for the Event monies and 2) the Financial type Donation for the extras;

That’s how it works for Memberships and top-up optional Donations (for sure - doing that on a number of projects). The Financial Type for the Member $ is what is defined in the Membership Type Config settings.

If this does not work like that for Event registrations - I think it should (it should pull the Financial Type from the Event config); One should be able to have one Contribution with three lineItems - each with a different financial type:


Member Dues
Donation
Event Fees


The first two in combo are working for sure. I’ve not yet tried other combinations. 

Keep me posted on what you find.