expressions - What's the proper response when someone says something modest and underrated about himself

Recently when I told a British colleague of mine that he sent me the right assets earlier he told me this:

Wow, me being efficient? Doesn't sound right

I understand that this is supposed to be modesty in a funny way but I'm not sure how to respond to this. And so I choose an awkward silence instead. We have the same thing in the my own culture but translating the proper response there wouldn't make any sense in English. I know I can respond like this:

Oh! You're being too modest

But I don't want to sound like I'm from 18th century. So what could be the proper response to this sentence?