When viewing a file in diffusion, you seem to get no highlighting for a file like README (sample), and text/x-c++ highlighting for README.md (sample, though epriestley just fixed this in syntax.filemap in master). It would be nice to be able to specify "highlight as Remarkup" or reStructuredText or whatever.
@avivey pointed out in IRC that you can do this when browsing a particular commit; each diff viewer in a commit has a View Options > Highlight As... dropdown where you can can override the default text highlighting (sample).
Perhaps there's a workaround where you can influence the highlighting by tweaking the diffusion URL, but the obvious ?highlight=remarkup didn't work.
Note this is just changing the highlighting of the file's text. T5698: Github-style Remarkup previews on a checked-in resource is to actually render the file differently.