File format extension

[Paul A. Rubin]

Is the extension field in Edit->Preferences->File formats documented 
anywhere?  I tried changing the extension for a clone of the PDF 
(pdflatex) format to 'pdf4' (because I was trying to open it in a 
non-Acrobat viewer, and the files were opening in Acrobat Reader 
regardless).  Turns out this doesn't work, because pdflatex gives the 
file a .pdf extension regardless.

I'm not sure if this is a bug or just how things work, because I can't 
find any documentation of whether the field is intended to let me 
specify the extension I want or just communicate the extension the file 
is going to have (outside my control).

Paul