func OpenReaderAt(r io.ReaderAt, size int64) (*File, error)
xlsx.OpenReaderAtio.ReaderAtmultipart.Fileio.ReaderAt
xlsx.OpenReaderAt
 var ( file multipart.File size int64 err error ) file, _,err = req.FormFile("key") // size = // Calculate size xlsx.OpenReaderAt(file,size)