Unfortunately, I was inaccurate and used not quite correct ways of
saving the attached file.
That is, if I saved the attachment with RC menu, everything works
But if I tried to view the attachment or save it by browser command,
then the embedded html file is displayed and stored incorrectly.
The Hex codes of ciryllic leters are different when I saved them with RC
and browser commands.
The fact that the RC command "save" is output as D0 A1 D0 BE D1 81 ,
when saved by the browser (and, probably, when displayed) has the value
D0 A0 D0 8E D0 A0.