Spreedly's docs do not mention anything. Edit: They do indeed. I plead illiteracy.
Paypal's docs say the card type is "MasterCard", but actually trying to authorize with that will get you a "Card type is invalid." response from Paypal.
What is the magic string you need to successfully charge a MasterCard? Examples I have tried which did not work are:
In case others come here looking for the other card type strings, here they all are: