What filesystem is Wubi?

Wubi itself isn't a filesystem. It is simply files put onto a Windows partition. If your wanting to convert it to a real partition, I'd suggest using ext4, as it is the default filesystem Ubuntu uses in a real install.

Its an image, booted from grub (which can boot raw disk images) chainloaded from NTLDR or bcd using whatever's ubuntu's preferred file system - this would mean ext3 or 4. However this probably would not be a major factor in conversion from image to drive.

On the other hand, the method that used to be 'preferred' for such moves. lvpm is depreciated, you'd likely need to use some flavor of online backup, remastering or DD, and all 3 possibilities should work regardless of your filesystem.