[GplexBuffers]
   If Seek() is not supported on the input stream and "Guess" is specified,
   fallback to the default system code page.

   Verify that the input stream supports Seek() before attempting to
   recognize a Preamble or Guess the encoding. Fallback to using "raw"
   bytes or the user's fallback encoding when Seek() is not supported.

   If Seek() is supported, always attempt to guess the encoding before
   using the user's fallback encoding.