using System;

using System.IO;

using System.Collections.Generic;

public class Program

{

    public static void Main()

    {

        byte[] bytes = new byte[] 

        {

            255,

            216,

            255

        };

        MemoryStream stream = new MemoryStream(bytes);

        Console.WriteLine("Is JPG, JPEG: {0}", IsJpg(stream));

        Console.WriteLine("Is PNG: {0}", IsPng(stream));

    }

    public static bool IsPng(MemoryStream stream) {     

        List<string> pngBytesSignature = new List<string> 

        {

            "89",

            "50",

            "4E", 

            "47",

            "0D",

            "0A",

            "1A",

            "0A"

        };

        stream.Seek(0, SeekOrigin.Begin);

        byte[] fileSignatureBytes = new byte[8];

        stream.Read(fileSignatureBytes, 0, 8);

        for(int i = 0; i < pngBytesSignature.Count; i++){

            string bit = fileSignatureBytes[i].ToString("X2");

            string signatureBit = pngBytesSignature[i];

            if(bit != signatureBit)

                return false;

        }

        return true;

    }

    public static bool IsJpg(MemoryStream stream) {

        List<string> jpgBytesSignature = new List<string> 

        {

            "FF",

            "D8",

            "FF"

        };

        stream.Seek(0, SeekOrigin.Begin);

        byte[] fileSignatureBytes = new byte[3];

        stream.Read(fileSignatureBytes, 0, 3);

        for(int i = 0; i < jpgBytesSignature.Count; i++){

            string bit = fileSignatureBytes[i].ToString("X2");

            string signatureBit = jpgBytesSignature[i];

            if(bit != signatureBit)

                return false;

        }

        return true;

    }

}