Try an example
What are we looking for?
For a AI file, we look for the byte sequence %PDF which identifies that it is a PDF file, which Illustrator uses. Try changing the name of an AI file to a PDF, and see that it is the same.
Try your own
Or enter your own URL to analyse the first six sectors of the file:
File contents analysis
File signature analysis (Header analysis)PDF file (Starts with %PDF)
Additional scanning signature analysis
Possible: [TIF files, sig: 4949] Count: 1
File hex characters
 25 50 44 46 2D 31 2E 35 0D 25 EF BF BD EF BF BD %PDF-1.5.%......