[XeTeX] What characters are allowed in filenames?

[Ari Meir Brodsky]

What characters does XeLaTeX allow in file names, and why do they seem to be
different from PdfLaTeX?  I've created a simple file named:

mainfile(parens).tex

It runs fine with both pdfLaTeX and LuaLaTeX, but when I try to run XeLaTeX
on it, I get the following console output:

--------------------------------------------------------
This is XeTeX, Version 3.1415926-2.4-0.9998 (MiKTeX 2.9)
entering extended mode
! I can't find file `mainfileparens.tex'.
<*> mainfile(parens).tex
                        
Please type another input file name: 
--------------------------------------------------------

Why does XeLaTeX not accept the same file names that the other engines
accept?

Thanks.
-Ari

-----------------------
Ari M. Brodsky
ari.brodsky at utoronto.ca