If all your taxes are paid in reality you have 3 years and you don't really have to file an extension. But I would suggest that you file an extension since it's very easy to do anyways.
I think if you filed the extension, you then have three years to claim the refund from the date that the extension extended your deadline to. So instead of having until April 18, 2020, you get until October 16, 2020.
Ugh, I think I made a mistake.
Added all 1099 INTs but I remembered that I also had a bonus from Santander last year.. no 1099 so I had forgotten about it.
Do I file a 1040X to get this sorted out? I already filed my taxes a while ago..
If Santander actually prepared the form for you (they should have if they paid you $10 or more and had your correct SSN) but you didn't receive it for some reason, you'll find it on the Wage and Income Transcript that you can request from the IRS. If it is not there, you might just want to skip the 1040X.