New Post: EOF In Header exception when reading .xlsm

Hi,
how do you solve this ? I have exactly the same problem creating a XSSFWorkbook reading from stream (xlsx source):
Here I'm reading from posted file in SharePoint (properties are SPItemEventProperties). There is no problem if I return a HSSFWorkbook with the same filestream.

 Stream fileStream = null;
            HttpPostedFile postedFile = null;
            string searchForFileName = Path.GetFileName(properties.BeforeUrl);
            HttpFileCollection collection = _context.Request.Files;
            for (int i = 0; i < collection.Count; i++)
            {
                postedFile = collection[i];
                if (searchForFileName.Equals(
                  Path.GetFileName(postedFile.FileName), StringComparison.OrdinalIgnoreCase))
                {
                    fileStream = postedFile.InputStream;
                    fileStream.Position = 0;
                    byte[] fileContents = new byte[postedFile.ContentLength];
                    fileStream.Read(fileContents, 0, postedFile.ContentLength);
                }
            }

            if (postedFile.FileName.ToLowerInvariant().EndsWith("xlsx"))
            {
                return new XSSFWorkbook(fileStream);

            }