[latexrefman] \openin & \openout

[Karl Berry]

There is no reliable (cross-platform, cross-implementation, backward
compatible, etc.) way to use spaces in filenames.  There are various
hacks with various pros and cons, but I see no need to describe them
here. If we say anything, I think it should be simply that using
anything beyond the basic ASCII letters and numbers is going to be
implementation-dependent. 

Same for what happens with extensions. Basically, Knuth created a morass
in this area from the beginning by (understandably) using the
conventions of his development machine instead of defining a
system-independent layer.  Hence subsequent craziness in all directions ... -k