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, but I really have no idea how to go about it. Im using c# and asp.net.

Thanks for any advice.

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ok, so from the above links I now know that I am looking for ‘ff d8 ff e0’ to positively identify a .jpg file for example.

In my code I can read the first twenty bytes no problem:

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

so (and please excuse my total inexperience here) but how do I check whether the byte array contains ‘ff d8 ff e0’?

Answer 1

Here’s a quick-and-dirty response to the followup question you posted:

byte[] jpg = new byte[] { 0xFF, 0xD8, 0xFF, 0xE0 }; bool match = true; for (int i = 0; i < jpg.Length; i++) {     if (jpg[i] != b[i])     {         match = false;         break;     } }