A small typo in diffutils.py leads to 

.c

Traceback (most recent call last):
  File "/home/aharvey/Code/reviewboard/reviewboard/diffviewer/views.py", line 153, in view_diff
    interdiffset, highlighting, True)
  File "/home/aharvey/Code/reviewboard/reviewboard/diffviewer/diffutils.py", line 880, in get_diff_files
    large_data=True)
  File "/usr/local/lib/python2.6/dist-packages/Djblets-0.5.7alpha0.dev-py2.6.egg/djblets/util/misc.py", line 162, in cache_memoize
    data = lookup_callable()
  File "/home/aharvey/Code/reviewboard/reviewboard/diffviewer/diffutils.py", line 879, in <lambda>
    enable_syntax_highlighting)),
  File "/home/aharvey/Code/reviewboard/reviewboard/diffviewer/diffutils.py", line 601, in get_chunks
    register_interesting_lines_for_filename(differ, file)
  File "/home/aharvey/Code/reviewboard/reviewboard/diffviewer/diffutils.py", line 375, in register_interesting_lines_for_filename
    regexes = HEADER_REGEXES[HEADER_REGEX_ALIASES[ext]]
KeyError: '.c'

The fixes is straight forward there is no .c in the diffutils.py, it looks like there was a typo, that the .c was set to .h in key.

Load a diff with a .cpp file, does not showup in the diff viewer, applied the attached diff, .cpp and .c now display correctly.