diff --git a/libs/filer/workspace_files_extensions_client.go b/libs/filer/workspace_files_extensions_client.go
index 44e21432c51..29adbdbddf6 100644
--- a/libs/filer/workspace_files_extensions_client.go
+++ b/libs/filer/workspace_files_extensions_client.go
@@ -124,6 +124,13 @@ func (w *WorkspaceFilesExtensionsClient) getNotebookStatByNameWithoutExt(ctx con
 		ext = notebook.ExtensionJupyter
 	}
 
+	// Extension-less notebook names cannot round-trip through [Stat] or [Read],
+	// which report them as not found, so return nil to have [ReadDir] omit the entry.
+	if ext == "" {
+		log.Warnf(ctx, "skipping notebook %s: no file extension is associated with notebook language %q", path.Join(w.root, name), stat.Language)
+		return nil, nil
+	}
+
 	// Modify the stat object path to include the extension. This stat object will be used
 	// to return the fs.DirEntry object in the ReadDir method.
 	stat.Path = stat.Path + ext
@@ -205,6 +212,7 @@ func (w *WorkspaceFilesExtensionsClient) ReadDir(ctx context.Context, name strin
 	}
 
 	seenPaths := make(map[string]workspace.ObjectInfo)
+	result := make([]fs.DirEntry, 0, len(entries))
 	for i := range entries {
 		info, err := entries[i].Info()
 		if err != nil {
@@ -218,6 +226,10 @@ func (w *WorkspaceFilesExtensionsClient) ReadDir(ctx context.Context, name strin
 			if err != nil {
 				return nil, err
 			}
+			// A nil stat means the notebook's language has no extension mapping; omit the entry.
+			if stat == nil {
+				continue
+			}
 			// Replace the entry with the new entry that includes the extension.
 			entries[i] = wsfsDirEntry{wsfsFileInfo{ObjectInfo: stat.ObjectInfo}}
 		}
@@ -232,9 +244,10 @@ func (w *WorkspaceFilesExtensionsClient) ReadDir(ctx context.Context, name strin
 			}
 		}
 		seenPaths[entries[i].Name()] = sysInfo
+		result = append(result, entries[i])
 	}
 
-	return entries, nil
+	return result, nil
 }
 
 // Note: The import API returns opaque internal errors for namespace clashes
diff --git a/libs/filer/workspace_files_extensions_client_test.go b/libs/filer/workspace_files_extensions_client_test.go
index b3b4861ee8e..438ac21ec47 100644
--- a/libs/filer/workspace_files_extensions_client_test.go
+++ b/libs/filer/workspace_files_extensions_client_test.go
@@ -201,3 +201,89 @@ func TestFilerWorkspaceFilesExtensionsErrorsOnDupName(t *testing.T) {
 		})
 	}
 }
+
+func TestFilerWorkspaceFilesExtensionsReadDirWithUnmappedLanguage(t *testing.T) {
+	for _, tc := range []struct {
+		name                 string
+		language             workspace.Language
+		notebookExportFormat workspace.ExportFormat
+		expectedNames        []string
+	}{
+		{
+			name:                 "source notebook with empty language is skipped",
+			language:             "",
+			notebookExportFormat: workspace.ExportFormatSource,
+			expectedNames:        []string{"bar.py"},
+		},
+		{
+			name:                 "source notebook with unknown language is skipped",
+			language:             workspace.Language("FUTURELANG"),
+			notebookExportFormat: workspace.ExportFormatSource,
+			expectedNames:        []string{"bar.py"},
+		},
+		{
+			name:                 "jupyter notebook with unknown language keeps the .ipynb extension",
+			language:             workspace.Language("FUTURELANG"),
+			notebookExportFormat: workspace.ExportFormatJupyter,
+			expectedNames:        []string{"bar.py", "foo.ipynb"},
+		},
+	} {
+		t.Run(tc.name, func(t *testing.T) {
+			mockedWorkspaceClient := mocks.NewMockWorkspaceClient(t)
+			mockedApiClient := mockApiClient{}
+
+			workspaceApi := mockedWorkspaceClient.GetMockWorkspaceAPI()
+			workspaceApi.EXPECT().ListAll(mock.Anything, workspace.ListWorkspaceRequest{
+				Path: "/dir",
+			}).Return([]workspace.ObjectInfo{
+				{
+					Path:       "/dir/bar.py",
+					ObjectType: workspace.ObjectTypeFile,
+				},
+				{
+					Path:       "/dir/foo",
+					Language:   tc.language,
+					ObjectType: workspace.ObjectTypeNotebook,
+				},
+			}, nil)
+
+			// Mock the get-status call used to figure out the notebook's file extension.
+			statNotebook := wsfsFileInfo{
+				ObjectInfo: workspace.ObjectInfo{
+					Path:       "/dir/foo",
+					Language:   tc.language,
+					ObjectType: workspace.ObjectTypeNotebook,
+				},
+				ReposExportFormat: tc.notebookExportFormat,
+			}
+
+			mockedApiClient.On("Do", mock.Anything, http.MethodGet, "/api/2.0/workspace/get-status", map[string]string(nil), map[string]string{
+				"path":               "/dir/foo",
+				"return_export_info": "true",
+			}, mock.AnythingOfType("*filer.wsfsFileInfo"), []func(*http.Request) error(nil)).Return(nil, statNotebook)
+
+			workspaceFilesClient := WorkspaceFilesClient{
+				workspaceClient: mockedWorkspaceClient.WorkspaceClient,
+				apiClient:       &mockedApiClient,
+				root:            NewWorkspaceRootPath("/dir"),
+			}
+
+			workspaceFilesExtensionsClient := WorkspaceFilesExtensionsClient{
+				workspaceClient: mockedWorkspaceClient.WorkspaceClient,
+				wsfs:            &workspaceFilesClient,
+			}
+
+			entries, err := workspaceFilesExtensionsClient.ReadDir(t.Context(), "/")
+			assert.NoError(t, err)
+
+			names := make([]string, len(entries))
+			for i, entry := range entries {
+				names[i] = entry.Name()
+			}
+			assert.Equal(t, tc.expectedNames, names)
+
+			workspaceApi.AssertNumberOfCalls(t, "ListAll", 1)
+			mockedApiClient.AssertNumberOfCalls(t, "Do", 1)
+		})
+	}
+}