diff --git a/src/agents/pi-tools.read.ts b/src/agents/pi-tools.read.ts
index 5de5019635b..3798c6dd8b1 100644
--- a/src/agents/pi-tools.read.ts
+++ b/src/agents/pi-tools.read.ts
@@ -3,6 +3,7 @@ import { createEditTool, createReadTool, createWriteTool } from "@mariozechner/p
 import type { AnyAgentTool } from "./pi-tools.types.js";
 import type { SandboxFsBridge } from "./sandbox/fs-bridge.js";
 import { detectMime } from "../media/mime.js";
+import { sniffMimeFromBase64 } from "../media/sniff-mime-from-base64.js";
 import { assertSandboxPath } from "./sandbox-paths.js";
 import { sanitizeToolResultImages } from "./tool-images.js";
 
@@ -12,26 +13,6 @@ type ToolContentBlock = AgentToolResult<unknown>["content"][number];
 type ImageContentBlock = Extract<ToolContentBlock, { type: "image" }>;
 type TextContentBlock = Extract<ToolContentBlock, { type: "text" }>;
 
-async function sniffMimeFromBase64(base64: string): Promise<string | undefined> {
-  const trimmed = base64.trim();
-  if (!trimmed) {
-    return undefined;
-  }
-
-  const take = Math.min(256, trimmed.length);
-  const sliceLen = take - (take % 4);
-  if (sliceLen < 8) {
-    return undefined;
-  }
-
-  try {
-    const head = Buffer.from(trimmed.slice(0, sliceLen), "base64");
-    return await detectMime({ buffer: head });
-  } catch {
-    return undefined;
-  }
-}
-
 function rewriteReadImageHeader(text: string, mimeType: string): string {
   // pi-coding-agent uses: "Read image file [image/png]"
   if (text.startsWith("Read image file [") && text.endsWith("]")) {
diff --git a/src/gateway/chat-attachments.ts b/src/gateway/chat-attachments.ts
index b9e78184032..bb1e2c2e7ab 100644
--- a/src/gateway/chat-attachments.ts
+++ b/src/gateway/chat-attachments.ts
@@ -1,5 +1,5 @@
 import { estimateBase64DecodedBytes } from "../media/base64.js";
-import { detectMime } from "../media/mime.js";
+import { sniffMimeFromBase64 } from "../media/sniff-mime-from-base64.js";
 
 export type ChatAttachment = {
   type?: string;
@@ -31,26 +31,6 @@ function normalizeMime(mime?: string): string | undefined {
   return cleaned || undefined;
 }
 
-async function sniffMimeFromBase64(base64: string): Promise<string | undefined> {
-  const trimmed = base64.trim();
-  if (!trimmed) {
-    return undefined;
-  }
-
-  const take = Math.min(256, trimmed.length);
-  const sliceLen = take - (take % 4);
-  if (sliceLen < 8) {
-    return undefined;
-  }
-
-  try {
-    const head = Buffer.from(trimmed.slice(0, sliceLen), "base64");
-    return await detectMime({ buffer: head });
-  } catch {
-    return undefined;
-  }
-}
-
 function isImageMime(mime?: string): boolean {
   return typeof mime === "string" && mime.startsWith("image/");
 }
diff --git a/src/media/sniff-mime-from-base64.ts b/src/media/sniff-mime-from-base64.ts
new file mode 100644
index 00000000000..631b08a9c80
--- /dev/null
+++ b/src/media/sniff-mime-from-base64.ts
@@ -0,0 +1,21 @@
+import { detectMime } from "./mime.js";
+
+export async function sniffMimeFromBase64(base64: string): Promise<string | undefined> {
+  const trimmed = base64.trim();
+  if (!trimmed) {
+    return undefined;
+  }
+
+  const take = Math.min(256, trimmed.length);
+  const sliceLen = take - (take % 4);
+  if (sliceLen < 8) {
+    return undefined;
+  }
+
+  try {
+    const head = Buffer.from(trimmed.slice(0, sliceLen), "base64");
+    return await detectMime({ buffer: head });
+  } catch {
+    return undefined;
+  }
+}