Everyone knows that credit cards were actually quite well-suited to the web, and so the credit card form was born. There are several sites who have done away with the credit card type field. Both Amazon and GitHub don’t require you to select a credit card type in their form. As you fill in the credit card number, the user interface changes to show the type of credit card being used.

Auto-detecting Credit Card Type is a nice addition to the standard payment form because it frees up the user from entering what is actually redundant information. It’s trivial to determine the type of credit card being used once you understand how credit card numbers work.

Source: http://webstandardssherpa.com/reviews/auto-detecting-credit-card-type/