The first FAT entry should have the media byte (0xf0,0xf8,...,0xff)
extended with all 1 bits in the first FAT entry.

Checking this is a good idea, it prevents us from mounting garbage
as FAT - there is no good magic for FAT.

Unfortunately, Windows does not enforce this, and 2.4 doesn't either.
It turns out that there are filesystems around (two reports so far) that
have a zero first FAT entry, and work under Windows and 2.4 but fail to
mount under 2.6.

So, this weakens the test.