Can you tell the exact purpose for which you are asking this question because I still find your question vague. File extension is highly subjective matter and differs from wallet to wallet, for example Electrum wallet file doesn't even have the extension. By the way, this page covers wallet formats for few notable wallets: https://en.bitcoin.it/wiki/Wallet

And what exactly do you mean by 'online wallet'? All wallets need to be online to broadcast the transaction. Or do you mean 'web wallet'? In that case, unlike blockchain.info, most of the web wallets are custodial wallets and don't create wallet file for users. They rather save users' keys in centralized database. Moreover, I doubt whether there were many notable web wallets in 2009-2012 period.

Anyhow, you need to be more clear with your question and tell us what exactly are you trying to achieve to get more relevant answers.

File extension has no relevance in bitcoin wallets. It can be any format. It's just a file which stores details like private key, public key etc. As a wallet developer, I can store keys and data in any format. It basically depend upon the security level and more on how my application is made and how it fetches the data for signing and verifying transactions.