Enable FileRef to detect file types by the actual content of a stream.

Enable FileRef to detect file types by the actual content of a stream.

FileRef doesn't work with ByteVectorStream as reported at #796, since ByteVectorStream is not associated with a file name and FileRef detects file types based on file extensions.
This commit makes FileRef to work with ByteVectorStream by enabling it to detect file types based on the actual content of a stream.