Something

  private long getContentEnd(FileInfo imgFile, long end, int testLength)
        {
            FileStream img = SupportClass.RandomAccessFileSupport.CreateRandomAccessFile(imgFile, "r"); ;
            byte[] bytes = { 255, 255, 255, 255, 255, 255, 255, 255,
                            255, 255, 255, 255, 255, 255, 255, 255,};
            int skip = bytes.Length;
            int remainTestLength = testLength - skip;
            byte[] testByte = new byte[skip];
            byte[] byte2 = new byte[remainTestLength];
            byte[] testByte2 = new byte[remainTestLength];

            for (int i = 0; i < remainTestLength; i++) 
                byte2[i] = 255; 

            for (long j = testLength; j < end; j += testLength)
            {
                img.Seek(j, SeekOrigin.Begin );

                Console.WriteLine(j);
                img.Read(testByte, 0, testByte.Length);                
                if ( bytes.Equals(testByte) )
                {
                    img.Read(testByte2, 0, testByte2.Length);
                    if (byte2.Equals(testByte2))
                    {
                        long k = 0L;

                        do
                        {
                            img.Read(testByte, 0, testByte.Length);
                            img.Read(testByte2, 0, testByte2.Length);

                            k += testLength;
                
                            if (k >= end)
                                return j;
                        }
                        while (k < end && bytes.Equals(testByte) && 
                            byte2.Equals( testByte2) );
                    }
                }
            }
            return end;
        }