reStructuredText is a very handy markup language, used for many things including Python module documentation: http://docutils.sourceforge.net/rst.html Its support should be easy (it has been added recently to gedit, for instance). It would be good to add it to mousepad also. I'm available fortesting and further info. Thanks.
Language highlighting is related to GtkSourceView[1]. If you take a look at GtkSourceView source[2], you'll see it has a 'rst.lang' file. It was added in 2013-11-15. The problem here is that mousepad uses gtksourceview2 which seems to be not updated. Fortunately you can manually add the RST language syntax[3]. [1] https://wiki.gnome.org/Projects/GtkSourceView/LanguageDefinitions [2] https://git.gnome.org/browse/gtksourceview/tree/data/language-specs [3] https://mail.gnome.org/archives/gedit-list/2011-May/msg00012.html
Closing this bug as the problem is related to GtkSourceView. The rst files will be supported out of the box when(and only if) mousepad is ported to gtk3 :/ Reopen if disagree.