diff --git a/frontend/src/core/utils/files.tsx b/frontend/src/core/utils/files.tsx
index 4c4d6ca116..97e0574109 100644
--- a/frontend/src/core/utils/files.tsx
+++ b/frontend/src/core/utils/files.tsx
@@ -109,7 +109,8 @@ const extensionMap: Record<string, string> = {
   lua: "lua",
   r: "r",
   matlab: "matlab",
-  julia: "jl",
+  julia: "julia",
+  jl: "julia",
   elm: "elm",
   haskell: "haskell",
   hs: "haskell",
diff --git a/frontend/tests/unit/core/utils/files.test.ts b/frontend/tests/unit/core/utils/files.test.ts
index f10aa44ad8..97a9d2a0d8 100644
--- a/frontend/tests/unit/core/utils/files.test.ts
+++ b/frontend/tests/unit/core/utils/files.test.ts
@@ -1,7 +1,11 @@
 import { expect, test } from "@rstest/core";
 import { FilePlayIcon, FileTextIcon, ImageIcon } from "lucide-react";
 
-import { canBrowserPreviewFile, getFileIcon } from "@/core/utils/files";
+import {
+  canBrowserPreviewFile,
+  checkCodeFile,
+  getFileIcon,
+} from "@/core/utils/files";
 
 test.each([
   ["animation.apng", ImageIcon],
@@ -15,3 +19,28 @@ test.each([
 test("keeps the document icon for an unknown extension", () => {
   expect(getFileIcon("data.unknown").type).toBe(FileTextIcon);
 });
+
+// `extensionMap` maps a file extension to a language name. Both halves of that
+// direction matter: `checkCodeFile` tests extension membership, and the language
+// is what the editor and previewers receive.
+test("resolves a language from its extension", () => {
+  expect(checkCodeFile("script.hs")).toEqual({
+    isCodeFile: true,
+    language: "haskell",
+  });
+  expect(checkCodeFile("deploy.ex")).toEqual({
+    isCodeFile: true,
+    language: "elixir",
+  });
+  expect(checkCodeFile("analysis.jl")).toEqual({
+    isCodeFile: true,
+    language: "julia",
+  });
+});
+
+test("values the language name, never the extension it came from", () => {
+  expect(checkCodeFile("notes.julia")).toEqual({
+    isCodeFile: true,
+    language: "julia",
+  });
+});