www.mutt.org/doc/manual/manual-5.html を読むと If the file is free of binary information, Mutt will assume that the file is plain text, and mark it as text/plain. If the file contains binary information, then Mutt will mark it as application/octet-stream. とありますが，application/octet-streamと認識されるテキストファイルも当然 バイナリは含んでいません．
$HOME/.mime.typesには text/plain asc txt text diff のエントリがあります．
