diff --git a/src/media/mime.test.ts b/src/media/mime.test.ts
index a3c2a35d8..92325a62e 100644
--- a/src/media/mime.test.ts
+++ b/src/media/mime.test.ts
@@ -1,7 +1,7 @@
 import JSZip from "jszip";
 import { describe, expect, it } from "vitest";
 
-import { detectMime, imageMimeFromFormat } from "./mime.js";
+import { detectMime, extensionForMime, imageMimeFromFormat } from "./mime.js";
 
 async function makeOoxmlZip(opts: { mainMime: string; partPath: string }): Promise<Buffer> {
   const zip = new JSZip();
@@ -53,3 +53,47 @@ describe("mime detection", () => {
     expect(mime).toBe("application/vnd.openxmlformats-officedocument.spreadsheetml.sheet");
   });
 });
+
+describe("extensionForMime", () => {
+  it("maps image MIME types to extensions", () => {
+    expect(extensionForMime("image/jpeg")).toBe(".jpg");
+    expect(extensionForMime("image/png")).toBe(".png");
+    expect(extensionForMime("image/webp")).toBe(".webp");
+    expect(extensionForMime("image/gif")).toBe(".gif");
+    expect(extensionForMime("image/heic")).toBe(".heic");
+  });
+
+  it("maps audio MIME types to extensions", () => {
+    expect(extensionForMime("audio/mpeg")).toBe(".mp3");
+    expect(extensionForMime("audio/ogg")).toBe(".ogg");
+    expect(extensionForMime("audio/x-m4a")).toBe(".m4a");
+    expect(extensionForMime("audio/mp4")).toBe(".m4a");
+  });
+
+  it("maps video MIME types to extensions", () => {
+    expect(extensionForMime("video/mp4")).toBe(".mp4");
+    expect(extensionForMime("video/quicktime")).toBe(".mov");
+  });
+
+  it("maps document MIME types to extensions", () => {
+    expect(extensionForMime("application/pdf")).toBe(".pdf");
+    expect(extensionForMime("text/plain")).toBe(".txt");
+    expect(extensionForMime("text/markdown")).toBe(".md");
+  });
+
+  it("handles case insensitivity", () => {
+    expect(extensionForMime("IMAGE/JPEG")).toBe(".jpg");
+    expect(extensionForMime("Audio/X-M4A")).toBe(".m4a");
+    expect(extensionForMime("Video/QuickTime")).toBe(".mov");
+  });
+
+  it("returns undefined for unknown MIME types", () => {
+    expect(extensionForMime("video/unknown")).toBeUndefined();
+    expect(extensionForMime("application/x-custom")).toBeUndefined();
+  });
+
+  it("returns undefined for null or undefined input", () => {
+    expect(extensionForMime(null)).toBeUndefined();
+    expect(extensionForMime(undefined)).toBeUndefined();
+  });
+});
diff --git a/src/media/mime.ts b/src/media/mime.ts
index 79677b1cb..c50e9152c 100644
--- a/src/media/mime.ts
+++ b/src/media/mime.ts
@@ -13,7 +13,10 @@ const EXT_BY_MIME: Record<string, string> = {
   "image/gif": ".gif",
   "audio/ogg": ".ogg",
   "audio/mpeg": ".mp3",
+  "audio/x-m4a": ".m4a",
+  "audio/mp4": ".m4a",
   "video/mp4": ".mp4",
+  "video/quicktime": ".mov",
   "application/pdf": ".pdf",
   "application/json": ".json",
   "application/zip": ".zip",