When viewing a file in diffusion, you seem to get no highlighting for a file like README ([[ https://phabricator.wikimedia.org/diffusion/ETMH/browse/master/README | sample ]]), and text/x-c++ highlighting for README.md ([[ https://phabricator.wikimedia.org/diffusion/ECTX/browse/master/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 ([[ https://phabricator.wikimedia.org/rETMHa4a14b#diff-c47c7c73 | 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} is to actually //render// the file differently.