Symptom:
Files in the diff aren't listed in alphabetical order.

Resolution:
In generate_files function of reviewboard.diffviewer.diffutils module, 
'files' list are sorted before it is returned based on basepath in asc order, 
filename in asc orde,  and extension in desc order.
It means that hello.c and hello.h are sorted to hello.h, hello.c order.
However README, README.en, README.ja are sorted to README.ja, README.en, README order.
I think it's acceptable. 
       

On my local server, submitted diff file which contains several files including some header files.
Checked diff viewer if files are sorted in expected order.