How can I get the FILE SIGNATURE?

the file header first 20 bytes

Then you need to get the signatures and look them up. Should get you started.

FileStream fs = File.Open(path, FileMode.Open);
Byte[] b = new byte[20];
fs.Read(b, 0, 20);

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Does all file have on header in first 20byte???

That will get you the filetype for the file without checking the extension. For example I cant send .exe in my email. Some users change the .exe to .txt to fake the filetype. Good firewalls and sensors don't rely on the .extension but will check the header and file checksum.  The first 20 bytes will get you the file type but with a hex editor or similar program. The checksum helps validate against this type of change. Of course to ensure the correct delivery you will see MD5 public hash keys to match the program to. Many open source programs will distribute the program with a md5 hash. The user will then use a md5 generator and confirm the match which validates that not one byte has been modified from the original.

Thaks for that info...

Can you give me a sample code for that.