It's just a binary (raw) version of the blockchain that can be imported using monero-blockchain-import. To my knowledge the only tool that can read this file is monero-blockchain-import.
The file extension is immaterial.
I'm pretty sure you're not going to be able to share a folder on Windows and mount it in a VirtualBox guest Ubuntu, using the blockchain from that mounted shared folder. This is because the blockchain file is memory mapped when opened and I'm pretty certain vboxsf (which is the kernel module that handles the share), doesn't support this.
You could create ...