Fix matrix media attachments (#12967) thanks @williamtwomey

Co-authored-by: Gustavo Madeira Santana @gumadeiras

extensions/matrix/src/matrix/monitor/media.ts

@@ -30,11 +30,13 @@ async function fetchMatrixMediaBuffer(params: {
   // Use the client's download method which handles auth
   try {
     const result = await params.client.downloadContent(params.mxcUrl);
-    const buffer = result.data;
+    const raw = result.data ?? result;
+    const buffer = Buffer.isBuffer(raw) ? raw : Buffer.from(raw);
+
     if (buffer.byteLength > params.maxBytes) {
       throw new Error("Matrix media exceeds configured size limit");
     }
-    return { buffer: Buffer.from(buffer) };
+    return { buffer, headerType: result.contentType };
   } catch (err) {
     throw new Error(`Matrix media download failed: ${String(err)}`, { cause: err });
   }