The external storage when on a SD card is FAT. Period. You are just going to cause yourself a mess if you try to do otherwise. The basic semantics of how external storage is used relies on it being fat -- no permissions, case insensitive, etc. (For "emulated" external storage where it is shared with the internal EXT4 or whatever partition, there is a special filesystem layer put on top to emulate the FAT semantics.)
The reason for SD cards being FAT, is because that is the standard that everything operates with.
You can of course mess around and try other things, but you are
going to have problems, this is not
supported, and you certainly shouldn't be asking on a post about Google Android mobile offerings about this stuff.