diff --git a/docbow_project/docbow/utils.py b/docbow_project/docbow/utils.py
index bcf92ab..f031418 100644
--- a/docbow_project/docbow/utils.py
+++ b/docbow_project/docbow/utils.py
@@ -41,7 +41,10 @@ def file_match_mime_types(file_like, mime_types):
 def mime_types_to_extensions(mime_types):
     extensions = set()
     for mime_type in mime_types:
-        extensions.update(mimetypes.guess_all_extensions(mime_type, strict=False))
+        extension = mimetypes.guess_extension(mime_type, strict=False)
+        if extension is None:
+            continue
+        extensions.add(extension)
     return sorted(extensions)