Nothing... for now

(削除) è (0x8A) tail control characters (削除ここまで) FIXED

These 1985, MS-DOS text has a 0x8A è lead and then a bunch of SUB controls. The next examples have \n then the series of SUB. They may have been introduced by some ancient BBS software?

https://defacto2.net/f/b22621c
https://defacto2.net/f/b328b2c

(削除) Unknown magic number detection (削除ここまで) FIXED

https://defacto2.net/f/bd2bf3e

It is occasionally treating plain text as "Unknown". In the first item, there is a SUB control character at the end of the text file. The issue is not the SUB, maybe there is a pattern in the text triggering it.
func Ansi(r io.ReaderAt) bool {} is returning true.