UNIX Command
$file vmlinuz-3.1.0-1-amd64 vmlinuz-3.1.0-1-amd64: Linux kernel x86 boot executable bzImage, version 3.1.0-1-amd64 (unknown@Debian) #1 SMP Tue Nov 29 13:47:12 UTC 2, RO-rootFS, swap_dev 0x2, Normal VGA $
UNIX Explanation
file tests each argument in an attempt to classify it. There are three sets of tests, performed in this order: filesystem tests, magic tests, and language tests. The first test that succeeds causes the file type to be printed.
Theory Drop . magic number
A constant numerical or text value used to identify a file format or protocol. source : http://en.wikipedia.org/wiki/Magic_number_(programming)