diff --git a/src/infra/outbound/message-action-params.attachments.test.ts b/src/infra/outbound/message-action-params.attachments.test.ts
index 2d539f0ab82c..264926efe193 100644
--- a/src/infra/outbound/message-action-params.attachments.test.ts
+++ b/src/infra/outbound/message-action-params.attachments.test.ts
@@ -29,6 +29,7 @@ const onePixelPng = Buffer.from(
 const onePixelPngBase64 = onePixelPng.toString("base64");
 const wrappedOnePixelPngBase64 = onePixelPngBase64.match(/.{1,24}/g)?.join("\r\n") ?? "";
 const parameterizedPngDataUrl = `data:image/png;charset=utf-8;name=../../ignored.svg;base64,${wrappedOnePixelPngBase64}`;
+const csvBase64 = Buffer.from("name,value\nexample,1\n").toString("base64");
 
 function firstMockArg(
   mock: { mock: { calls: readonly unknown[][] } },
@@ -106,10 +107,29 @@ describe("runMessageAction media behavior", () => {
     await resetMessageActionMediaMocks();
   });
 
-  it.each(["send", "sendAttachment", "reply", "upload-file", "setGroupIcon"] as const)(
-    "normalizes parameterized, line-wrapped image data URLs for %s",
-    async (action) => {
-      const args: Record<string, unknown> = { buffer: parameterizedPngDataUrl };
+  it.each(
+    (["send", "sendAttachment", "reply", "upload-file", "setGroupIcon"] as const).flatMap(
+      (action) => [
+        {
+          action,
+          buffer: parameterizedPngDataUrl,
+          base64: onePixelPngBase64,
+          contentType: "image/png",
+          filename: "attachment.png",
+        },
+        {
+          action,
+          buffer: `data:text/csv;base64,${csvBase64}`,
+          base64: csvBase64,
+          contentType: "text/csv",
+          filename: "attachment.csv",
+        },
+      ],
+    ),
+  )(
+    "normalizes $contentType data URLs and infers filenames for $action",
+    async ({ action, buffer, base64, contentType, filename }) => {
+      const args: Record<string, unknown> = { buffer };
 
       await hydrateAttachmentParamsForAction({
         cfg: {},
@@ -120,14 +140,13 @@ describe("runMessageAction media behavior", () => {
         mediaPolicy: { mode: "host" },
       });
 
-      expect(args.contentType).toBe("image/png");
+      expect(args.contentType).toBe(contentType);
+      expect(args.filename).toBe(filename);
       if (action === "send") {
         expect(args.media).toBe("buffer://message-send/attachment");
-        expect(args.filename).toBe("attachment.png");
       } else {
-        expect(canonicalizeBase64(String(args.buffer))).toBe(onePixelPngBase64);
+        expect(canonicalizeBase64(String(args.buffer))).toBe(base64);
       }
-      expect(args.filename).not.toBe("../../ignored.svg");
     },
   );
 
@@ -329,6 +348,7 @@ describe("runMessageAction media behavior", () => {
 
         const payload = requireActionPayload(result);
         expect(payload.contentType).toBe("image/png");
+        expect(payload.filename).toBe("attachment.png");
         expect(canonicalizeBase64(String(payload.buffer))).toBe(onePixelPngBase64);
       },
     );
@@ -617,6 +637,7 @@ describe("runMessageAction media behavior", () => {
 
       const handlerParams = firstMockArg(handleActionMock, "handleAction");
       expect(handlerParams.contentType).toBe("image/png");
+      expect(handlerParams.filename).toBe("attachment.png");
       expect(canonicalizeBase64(String(handlerParams.buffer))).toBe(onePixelPngBase64);
     });
 
diff --git a/src/infra/outbound/message-action-params.ts b/src/infra/outbound/message-action-params.ts
index 920ea77d6b1d..e19135cf293e 100644
--- a/src/infra/outbound/message-action-params.ts
+++ b/src/infra/outbound/message-action-params.ts
@@ -538,7 +538,7 @@ async function hydrateAttachmentPayload(params: {
   } else if (!filename) {
     params.args.filename = inferAttachmentFilename({
       mediaHint: mediaSource,
-      contentType: contentTypeParam ?? undefined,
+      contentType: normalized.contentType,
     });
   }
 }