\documentclass{article}

\usepackage{expl3}
\begin{document}

\ExplSyntaxOn

\typeout{[\file_full_name:n{a~b}]}
\typeout{[\file_full_name:n{zzz-noext}]}
\typeout{[\file_full_name:n{space~here/abc}]}

\ExplSyntaxOff

\end{document}

$ ls space\ here/
abc.tex

[a b.tex]
[zzz-noext]
[abc.tex]

\typeout{[\file_full_name:n{space~here/grüße}]}
\typeout{[\file_full_name:n{grüße2}]}

[a b.tex]
[zzz-noext]
[abc]
[]
[]

[a b.tex]
[zzz-noext]
[abc]
[grüße]
[grüße2.tex]

\directlua{
  print(kpse.find_file("grüße2","tex",true))
}
\bye

$ luatex ef2
This is LuaTeX, Version 1.10.0 (TeX Live 2019)
 restricted system commands enabled.
(./ef2.tex./grüße2.tex
)
warning  (pdf backend): no pages of output.
Transcript written on ef2.log.

\directlua{
  print(kpse.find_file("grüße2","tex",true))
}
\bye

C:\Users\joseph\Desktop\test>kpsewhich grüße2.tex
./gr³▀e2.tex

C:\Users\joseph\Desktop\test>lualatex \input ./gr³▀e2.tex
This is LuaTeX, Version 1.10.0 (TeX Live 2019/W32TeX)
 restricted system commands enabled.
LaTeX2e <2019-10-01> patch level 1

luaotfload | main : initialization completed in 0.510 seconds
! String contains an invalid utf-8 sequence.
<*> \input ./gr
            e2.tex
?

This is LuaTeX, Version 1.10.0 (TeX Live 2019/W32TeX)
 restricted system commands enabled.
(./test.tex
LaTeX2e <2019-10-01> patch level 1

luaotfload | main : initialization completed in 0.583 seconds

! LaTeX Error: File `grüße2.tex' not found.

Type X to quit or <RETURN> to proceed,
or enter new name. (Default extension: tex)

Enter file name:

! fontspec error: "font-not-found"
!
! The font "academicons" cannot be found.

\usepackage{fontspec}

\newfontfamily{\AI}{academicons}

$ ls
ifetex.sty    ifptex.sty  ifuptex.sty  ifxetex.sty
ifluatex.sty  iftex.sty   ifvtex.sty   ifxptex.sty


$ cat *| wc -l
1204