1 files changed, 13 insertions, 1 deletions
diff --git a/sphinx/util/__init__.py b/sphinx/util/__init__.py
index d44290b7b..2032b94ac 100644
--- a/sphinx/util/__init__.py
+++ b/sphinx/util/__init__.py
@@ -29,7 +29,9 @@ from typing import Any, Callable, Dict, IO, Iterable, Iterator, List, Pattern, S
 from urllib.parse import urlsplit, urlunsplit, quote_plus, parse_qsl, urlencode
 
 from sphinx.deprecation import RemovedInSphinx40Warning
-from sphinx.errors import PycodeError, SphinxParallelError, ExtensionError
+from sphinx.errors import (
+    PycodeError, SphinxParallelError, ExtensionError, FiletypeNotFoundError
+)
 from sphinx.locale import __
 from sphinx.util import logging
 from sphinx.util.console import strip_colors, colorize, bold, term_width_line  # type: ignore
@@ -117,6 +119,16 @@ def get_matching_docs(dirname: str, suffixes: List[str],
                 break
 
 
+def get_filetype(source_suffix, filename):
+    # type: (Dict[str, str], str) -> str
+    for suffix, filetype in source_suffix.items():
+        if filename.endswith(suffix):
+            # If default filetype (None), considered as restructuredtext.
+            return filetype or 'restructuredtext'
+    else:
+        raise FiletypeNotFoundError
+
+
 class FilenameUniqDict(dict):
     """
     A dictionary that automatically generates unique names for its keys,