gistfile1.txt

public static class ContentTypeIdentifier
    {
        public static ContentType IdentifyType(Stream contentStream)
        {
            byte[] contentHeader = new byte[4];
            contentStream.Read(contentHeader, 0, 4);

            return ResolveContentTypeBasedOnHeader(contentHeader);
        }

        private static ContentType ResolveContentTypeBasedOnHeader(byte[] header)
        {
            var jpg = new byte[] { 255, 216, 255, 224 };
            var jpg2 = new byte[] { 255, 216, 255, 225 };
            var png = new byte[] { 137, 80, 78, 71 };
            var bmp = new byte[] { 66, 77 };
            var gif = new byte[] { 71, 73, 70 };

            if (jpg.SequenceEqual(header.Take(jpg.Length)))
                return ContentType.Jpg;

            if (jpg2.SequenceEqual(header.Take(jpg2.Length)))
                return ContentType.Jpg;

            if (png.SequenceEqual(header.Take(png.Length)))
                return ContentType.Png;

            if (bmp.SequenceEqual(header.Take(bmp.Length)))
                return ContentType.Bmp;

            if (gif.SequenceEqual(header.Take(gif.Length)))
                return ContentType.Gif;

            return ContentType.NotImage;
        } 
    }

    public enum ContentType
    {
        Jpg,
        Png,
        Bmp,
        Gif,
        NotImage
    }