luatex problem with file with umlauts on windows 10

[Akira Kakuto]

Dear Ulrike,

> Currently the file search in expl3 is broken because of this, so
> I'm considering our options

f = chgstrcp.utf8tosyscp(file)
    (file: utf-8, f: local code page)

f = chgstrcp.syscptoutf8(file)
    (file: local code page, f: utf-8)

work only in lua[hb]tex for TeX Live Windows.
Please use them in expl3, if they are usable.

Best,
Akira