How to check the content of an uploaded file without relying on its extension?

Question

How do you go about verifying the type of an uploaded file reliably without using the extension? I\'m guessing that you have to examine the header / read some of the bytes,

Answer 1

The first few bytes of a file will often tell you the file type. See, for example,
http://www.garykessler.net/library/file_sigs.html
http://www.astro.keele.ac.uk/oldusers/rno/Computing/File_magic.html

Use System.IO to read the byes as binary after the upload.

I'm curious, though, why you can't rely on on the ContentType header?