From the unetbootin source on Launchpad I believe it is formatted as a EXT2 file. See approximately line 4126 in src/unetbootin/unetbootin.cpp in the source from the project git repository.
I gather that on various setups this can either be a file or a separate file system.
I think the way this is done is sort of standard. I'm a bit more familiar with sticks created by Startup Disk Creator. The main and first partition (maybe only partition) has MS formatting. Unless there is a separate partition labeled casper-rw there is a casper-rw file, which itself is a disk image of an EXT file system (for unetbootin EXT2, directions for other live boot sticks have examples of EXT3 and EXT4).
You shouldn't necessarily expect to see much on this file system. It only contains files in the live systems / file system that have been changed or created. There's some magic that uses a special file system that overlays the read-only root file system from the CD with the changes made to the file system embeded on the casper-rw file.
On one of my sticks made last year with Startup Disk Creator I was able to mount the DOS file system on the stick and enter the commands:
sudo losetup /dev/loop0 casper-rw
sudo mount /dev/loop0 /tmp/a
and examine the contents of /tmp/a to see the files created or changed when running from the memory stick.