Make sure the file is what it seems...

FileMagicNumberHelper.cs

namespace Awesome.Csharp.Utilities.EveryoneNeeds
{
    public static class FileMagicNumberHelper
    {
        public static bool CheckUploadedFileMagicNumber(byte[] file)
        {
            //No file
            if (file == null) return false;
            //File contains less than 2 bytes
            if (file.Count() < 2) return false;

            //Read first byte
            var firstByte = file[0].ToString();
            //Read second byte
            var secondByte = file[1].ToString();

            var fileClass = string.Concat(firstByte, secondByte);

            var isFileJpg = (fileClass == "255216");
            var isFilePdf = (fileClass == "3780");
            var isFileDocx = (fileClass == "8075");//valid for docx, dotx and docm.
            if (!isFileJpg && !isFilePdf && !isFileDocx)
            {
                return false;
            }

            //File is valid!
            return true;
        }
    }
}