In addition to GFM, GitHub supports a number of other markup formats, including Textile, Perl Plain Old Documentation, Emacs Org-mode, and ReStructured Text.
Though no support is explicitly listed for .wiki
files, there is support for Mediawiki syntax in .mediawiki
files. If I had to guess, I'd guess that .wiki
files also use Mediawiki syntax.
Edit:
The OP has confirmed that .wiki
files use Mediawiki syntax.