diff --git a/doc/utils/sphinx-config/conf.py.in b/doc/utils/sphinx-config/conf.py.in
index 337485e689..4dda51b09c 100644
--- a/doc/utils/sphinx-config/conf.py.in
+++ b/doc/utils/sphinx-config/conf.py.in
@@ -69,7 +69,7 @@ images_config = {
 templates_path = ['_templates']
 
 # The suffix of source filenames.
-source_suffix = '.rst'
+source_suffix = {'.rst': 'restructuredtext'}
 
 # The encoding of source files.
 #source_encoding = 'utf-8-sig'