How does one disable (UTF-8?) input "fixup"?

[Winston]

What mode/variable controls text conversion/fixup when reading a file?

I have a mostly ASCII text file that contains a few stray non-ASCII
characters.

When the file is named "foo", doing "emacs foo" and searching shows
that the non-ASCII characters have been changed (from, for example,
\342\200\246 to \u7AB6\uFF66).

However, if I rename the file "foo.exe", do "emacs foo.exe" and search,
I see the original characters (no change).

In both cases, the mode showed (Fundamental).

Curiously, after "emacs foo.exe", I did find-file-other-window to read
in "foo", and the characters were NOT altered.

What controls whether that conversion is done or not?  Is there an easy
way to turn it on/off?  I didn't see anything in the results from
apropos "-mode" that looked relevant.

Thanks in advance,
 -WBE