diff --git a/CHANGELOG.md b/CHANGELOG.md
index 9895e404b..310d438e4 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -302,6 +302,7 @@ Docs: https://docs.openclaw.ai
 - Nodes/system.run approval binding: carry prepared approval plans through gateway forwarding and bind interpreter-style script operands across approval to execution, so post-approval script rewrites are denied while unchanged approved script runs keep working. Thanks @tdjackey for reporting.
 - Nodes/system.run PowerShell wrapper parsing: treat `pwsh`/`powershell` `-EncodedCommand` forms as shell-wrapper payloads so allowlist mode still requires approval instead of falling back to plain argv analysis. Thanks @tdjackey for reporting.
 - Control UI/auth error reporting: map generic browser `Fetch failed` websocket close errors back to actionable gateway auth messages (`gateway token mismatch`, `authentication failed`, `retry later`) so dashboard disconnects stop hiding credential problems. Landed from contributor PR #28608 by @KimGLee. Thanks @KimGLee.
+- Media/mime unknown-kind handling: return `undefined` (not `"unknown"`) for missing/unrecognized MIME kinds and use document-size fallback caps for unknown remote media, preventing phantom `<media:unknown>` Signal events from being treated as real messages. (#39199) Thanks @nicolasgrasset.
 
 ## 2026.3.2
 
diff --git a/src/media/constants.ts b/src/media/constants.ts
index 5dec8cedb..d87dafebc 100644
--- a/src/media/constants.ts
+++ b/src/media/constants.ts
@@ -3,11 +3,11 @@ export const MAX_AUDIO_BYTES = 16 * 1024 * 1024; // 16MB
 export const MAX_VIDEO_BYTES = 16 * 1024 * 1024; // 16MB
 export const MAX_DOCUMENT_BYTES = 100 * 1024 * 1024; // 100MB
 
-export type MediaKind = "image" | "audio" | "video" | "document" | "unknown";
+export type MediaKind = "image" | "audio" | "video" | "document";
 
-export function mediaKindFromMime(mime?: string | null): MediaKind {
+export function mediaKindFromMime(mime?: string | null): MediaKind | undefined {
   if (!mime) {
-    return "unknown";
+    return undefined;
   }
   if (mime.startsWith("image/")) {
     return "image";
@@ -27,7 +27,7 @@ export function mediaKindFromMime(mime?: string | null): MediaKind {
   if (mime.startsWith("application/")) {
     return "document";
   }
-  return "unknown";
+  return undefined;
 }
 
 export function maxBytesForKind(kind: MediaKind): number {
diff --git a/src/media/mime.test.ts b/src/media/mime.test.ts
index 3fd287331..cdc05016c 100644
--- a/src/media/mime.test.ts
+++ b/src/media/mime.test.ts
@@ -128,7 +128,9 @@ describe("mediaKindFromMime", () => {
     { mime: "text/plain", expected: "document" },
     { mime: "text/csv", expected: "document" },
     { mime: "text/html; charset=utf-8", expected: "document" },
-    { mime: "model/gltf+json", expected: "unknown" },
+    { mime: "model/gltf+json", expected: undefined },
+    { mime: null, expected: undefined },
+    { mime: undefined, expected: undefined },
   ] as const)("classifies $mime", ({ mime, expected }) => {
     expect(mediaKindFromMime(mime)).toBe(expected);
   });
@@ -136,4 +138,9 @@ describe("mediaKindFromMime", () => {
   it("normalizes MIME strings before kind classification", () => {
     expect(kindFromMime(" Audio/Ogg; codecs=opus ")).toBe("audio");
   });
+
+  it("returns undefined for missing or unrecognized MIME kinds", () => {
+    expect(kindFromMime(undefined)).toBeUndefined();
+    expect(kindFromMime("model/gltf+json")).toBeUndefined();
+  });
 });
diff --git a/src/media/mime.ts b/src/media/mime.ts
index fced9c612..e551350c0 100644
--- a/src/media/mime.ts
+++ b/src/media/mime.ts
@@ -187,6 +187,6 @@ export function imageMimeFromFormat(format?: string | null): string | undefined
   }
 }
 
-export function kindFromMime(mime?: string | null): MediaKind {
+export function kindFromMime(mime?: string | null): MediaKind | undefined {
   return mediaKindFromMime(normalizeMimeType(mime));
 }
diff --git a/src/telegram/send.ts b/src/telegram/send.ts
index 61292f666..88b9df19d 100644
--- a/src/telegram/send.ts
+++ b/src/telegram/send.ts
@@ -576,7 +576,8 @@ export async function sendMessageTelegram(
       fileName: media.fileName,
     });
     const isVideoNote = kind === "video" && opts.asVideoNote === true;
-    const fileName = media.fileName ?? (isGif ? "animation.gif" : inferFilename(kind)) ?? "file";
+    const fileName =
+      media.fileName ?? (isGif ? "animation.gif" : inferFilename(kind ?? "document")) ?? "file";
     const file = new InputFile(media.buffer, fileName);
     let caption: string | undefined;
     let followUpText: string | undefined;
diff --git a/src/web/media.test.ts b/src/web/media.test.ts
index 9db06e302..27a7d6ccb 100644
--- a/src/web/media.test.ts
+++ b/src/web/media.test.ts
@@ -457,7 +457,7 @@ describe("local media root guard", () => {
       }),
     ).resolves.toEqual(
       expect.objectContaining({
-        kind: "unknown",
+        kind: undefined,
       }),
     );
 
@@ -468,7 +468,7 @@ describe("local media root guard", () => {
       }),
     ).resolves.toEqual(
       expect.objectContaining({
-        kind: "unknown",
+        kind: undefined,
       }),
     );
   });
@@ -498,7 +498,7 @@ describe("local media root guard", () => {
       }),
     ).resolves.toEqual(
       expect.objectContaining({
-        kind: "unknown",
+        kind: undefined,
       }),
     );
   });
diff --git a/src/web/media.ts b/src/web/media.ts
index 1e0842bb7..200a2b033 100644
--- a/src/web/media.ts
+++ b/src/web/media.ts
@@ -19,7 +19,7 @@ import { resolveUserPath } from "../utils.js";
 export type WebMediaResult = {
   buffer: Buffer;
   contentType?: string;
-  kind: MediaKind;
+  kind: MediaKind | undefined;
   fileName?: string;
 };
 
@@ -284,12 +284,12 @@ async function loadWebMediaInternal(
   const clampAndFinalize = async (params: {
     buffer: Buffer;
     contentType?: string;
-    kind: MediaKind;
+    kind: MediaKind | undefined;
     fileName?: string;
   }): Promise<WebMediaResult> => {
     // If caller explicitly provides maxBytes, trust it (for channels that handle large files).
     // Otherwise fall back to per-kind defaults.
-    const cap = maxBytes !== undefined ? maxBytes : maxBytesForKind(params.kind);
+    const cap = maxBytes !== undefined ? maxBytes : maxBytesForKind(params.kind ?? "document");
     if (params.kind === "image") {
       const isGif = params.contentType === "image/gif";
       if (isGif || !optimizeImages) {
@@ -324,7 +324,7 @@ async function loadWebMediaInternal(
   if (/^https?:\/\//i.test(mediaUrl)) {
     // Enforce a download cap during fetch to avoid unbounded memory usage.
     // For optimized images, allow fetching larger payloads before compression.
-    const defaultFetchCap = maxBytesForKind("unknown");
+    const defaultFetchCap = maxBytesForKind("document");
     const fetchCap =
       maxBytes === undefined
         ? defaultFetchCap