[autosummary] fix suffix detection (#12149)

Fix a bug whereby the wrong file extension may be used,
when multiple suffixes are specified in the `source_suffix` configuration.

Co-authored-by: Chris Sewell <chrisj_sewell@hotmail.com>

CHANGES.rst

@@ -127,6 +127,10 @@ Bugs fixed
   Patch by James Addison.
 * #11578: HTML Search: Order non-main index entries after other results.
   Patch by Brad King.
+* #12147: autosummary: Fix a bug whereby the wrong file extension
+  may be used,
+  when multiple suffixes are specified in :confval:`source_suffix`.
+  Patch by Sutou Kouhei.
 
 Testing
 -------

sphinx/ext/autosummary/__init__.py

@@ -770,7 +770,7 @@ def run(self) -> tuple[list[Node], list[system_message]]:
 
 def get_rst_suffix(app: Sphinx) -> str | None:
     def get_supported_format(suffix: str) -> tuple[str, ...]:
-        parser_class = app.registry.get_source_parsers().get(suffix)
+        parser_class = app.registry.get_source_parsers().get(suffix.removeprefix('.'))
         if parser_class is None:
             return ('restructuredtext',)
         return parser_class.supported