fix: append file extension from MIME type in WhatsApp document fallback

Nic-vdwalt · claude · Nic-vdwalt · commit 591270bfdf0e · 2026-03-17T07:06:31.000+02:00
When sending documents without an explicit filename, the recipient
received files named "file" with no extension. Use extensionForMime()
to derive the correct extension from the MIME type in both the
auto-reply and outbound send paths.

Co-Authored-By: Claude Opus 4.6 &lt;noreply@anthropic.com&gt;
diff --git a/src/web/auto-reply/deliver-reply.ts b/src/web/auto-reply/deliver-reply.ts
@@ -6,6 +6,8 @@ import { logVerbose, shouldLogVerbose } from "../../globals.js";
 import { convertMarkdownTables } from "../../markdown/tables.js";
 import { markdownToWhatsApp } from "../../markdown/whatsapp.js";
 import { sleep } from "../../utils.js";
+import path from "node:path";
+import { extensionForMime } from "../../media/mime.js";
 import { loadWebMedia } from "../media.js";
 import { newConnectionId } from "../reconnect.js";
 import { formatError } from "../session.js";
@@ -138,8 +140,9 @@ export async function deliverWebReply(params: {
           "media:video",
         );
       } else {
-        const fileName = media.fileName ?? mediaUrl.split("/").pop() ?? "file";
         const mimetype = media.contentType ?? "application/octet-stream";
+        const rawName = media.fileName ?? mediaUrl.split("/").pop() ?? "file";
+        const fileName = path.extname(rawName) ? rawName : `${rawName}${extensionForMime(mimetype) ?? ""}`;
         await sendWithRetry(
           () =>
             msg.sendMedia({
diff --git a/src/web/inbound/send-api.test.ts b/src/web/inbound/send-api.test.ts
@@ -38,17 +38,31 @@ describe("createWebSendApi", () => {
     });
   });
 
-  it("falls back to default document filename when fileName is absent", async () => {
+  it("falls back to default document filename with extension when fileName is absent", async () => {
     const payload = Buffer.from("pdf");
     await api.sendMessage("+1555", "doc", payload, "application/pdf");
     expect(sendMessage).toHaveBeenCalledWith(
       "1555@s.whatsapp.net",
       expect.objectContaining({
         document: payload,
-        fileName: "file",
+        fileName: "file.pdf",
         caption: "doc",
         mimetype: "application/pdf",
       }),
     );
   });
+
+  it("derives extension from MIME type dynamically (text/csv)", async () => {
+    const payload = Buffer.from("a,b,c");
+    await api.sendMessage("+1555", "csv data", payload, "text/csv");
+    expect(sendMessage).toHaveBeenCalledWith(
+      "1555@s.whatsapp.net",
+      expect.objectContaining({
+        document: payload,
+        fileName: "file.csv",
+        caption: "csv data",
+        mimetype: "text/csv",
+      }),
+    );
+  });
 });
diff --git a/src/web/inbound/send-api.ts b/src/web/inbound/send-api.ts
@@ -1,6 +1,7 @@
 import type { AnyMessageContent, WAPresence } from "@whiskeysockets/baileys";
 import type { ActiveWebSendOptions } from "../active-listener.js";
 import { recordChannelActivity } from "../../infra/channel-activity.js";
+import { extensionForMime } from "../../media/mime.js";
 import { toWhatsappJid } from "../../utils.js";
 
 export function createWebSendApi(params: {
@@ -38,7 +39,7 @@ export function createWebSendApi(params: {
             ...(gifPlayback ? { gifPlayback: true } : {}),
           };
         } else {
-          const fileName = sendOptions?.fileName?.trim() || "file";
+          const fileName = sendOptions?.fileName?.trim() || `file${extensionForMime(mediaType) ?? ""}`;
           payload = {
             document: mediaBuffer,
             fileName,
