fix: use detected media type for screenshots instead of hard-coded image/png

browser_screenshot declared every capture as image/png regardless of the
actual bytes. On Chromium/Edge builds where captureVisibleTab returns JPEG
bytes, the stored attachment reference recorded mediaType "image/png" against
JPEG bytes, so dsh-attachment-local's read-time metadata verification fails
with ATTACHMENT_CORRUPT ("Stored attachment metadata does not match its
reference"), surfacing as code UNKNOWN and triggering endless LLM retries.

- image.ts: sniff the capture's magic bytes (PNG/JPEG/WebP/GIF) and declare
  that real media type to attachments.saveImage, instead of always "image/png".
- tools.ts: assemble the tool result from ref.mediaType (the store's verified
  type) rather than re-declaring "image/png" as const, and relax the output
  schema's const to the accepted media-type enum.
- tests: add a regression case asserting JPEG capture bytes surface as
  image/jpeg.

Co-Authored-By: DeepSeek Harness <noreply@deepseek.com>

packages/dsh-plugin-browserskill/src/image.ts

@@ -1,10 +1,11 @@
 /**
- * Screenshot delivery. The canonical tool value stays plain JSON (a PNG file
- * path plus pixel metadata); when the host mounts a durable attachment store
- * AND the calling route declares image input, the PNG bytes are additionally
- * committed through `ctx.attachments` so the render step can attach the image
- * itself to the tool result. Any uncertainty (no store, unknown route,
- * text-only model) falls back to the path-only form.
+ * Screenshot delivery. The canonical tool value stays plain JSON (a screenshot
+ * file path plus pixel metadata); when the host mounts a durable attachment
+ * store AND the calling route declares image input, the capture bytes are
+ * additionally committed through `ctx.attachments` so the render step can
+ * attach the image itself to the tool result. Any uncertainty (no store,
+ * unknown route, text-only model, unrecognized bytes) falls back to the
+ * path-only form.
  */
 
 import type { Context } from "@deepseek-ai/cordis";
@@ -33,6 +34,54 @@ interface LlmLike {
   ): Promise<{ inputModalities?: readonly string[] }>;
 }
 
+/**
+ * Sniff the image media type from magic bytes. The capture path can hand back
+ * JPEG (e.g. `chrome.tabs.captureVisibleTab` on some Chromium/Edge builds
+ * returning JPEG regardless of the requested format), so the declared type can
+ * never be trusted from the file extension alone.
+ * @param data - first bytes of the capture.
+ * @returns the detected media type, or undefined when unrecognized.
+ */
+function sniffImageMediaType(data: Uint8Array): string | undefined {
+  if (data.length >= 4) {
+    // PNG signature: 89 50 4E 47 (0D 0A 1A 0A)
+    if (data[0] === 0x89 && data[1] === 0x50 && data[2] === 0x4e && data[3] === 0x47) {
+      return "image/png";
+    }
+    // JPEG SOI: FF D8 FF
+    if (data[0] === 0xff && data[1] === 0xd8 && data[2] === 0xff) {
+      return "image/jpeg";
+    }
+    // WebP: "RIFF" (52 49 46 46) + "WEBP" at offset 8
+    if (
+      data[0] === 0x52 &&
+      data[1] === 0x49 &&
+      data[2] === 0x46 &&
+      data[3] === 0x46 &&
+      data.length >= 12 &&
+      data[8] === 0x57 &&
+      data[9] === 0x45 &&
+      data[10] === 0x42 &&
+      data[11] === 0x50
+    ) {
+      return "image/webp";
+    }
+    // GIF: "GIF87a" or "GIF89a"
+    if (
+      data[0] === 0x47 &&
+      data[1] === 0x49 &&
+      data[2] === 0x46 &&
+      data[3] === 0x38 &&
+      data.length >= 6 &&
+      (data[4] === 0x37 || data[4] === 0x39) &&
+      data[5] === 0x61
+    ) {
+      return "image/gif";
+    }
+  }
+  return undefined;
+}
+
 /**
  * Commit screenshot bytes to the host attachment store when the composition
  * supports durable images on the current model route.
@@ -46,7 +95,14 @@ export async function trySaveScreenshot(
 ): Promise<ImageAttachmentRef | undefined> {
   const attachments = ctx.get("attachments") as AttachmentLike | undefined;
   if (attachments === undefined) return undefined;
-  if (!attachments.imageLimits.mediaTypes.includes("image/png")) return undefined;
+  // Declare the real media type from the bytes, never a hard-coded "image/png".
+  // Declaring the wrong type trips the store's IMAGE_TYPE_MISMATCH check and
+  // silently degrades to path-only; sniffing first lets JPEG captures (returned
+  // by some Chromium builds for `captureVisibleTab({ format: "png" })`) still
+  // inline correctly.
+  const mediaType = sniffImageMediaType(data);
+  if (mediaType === undefined) return undefined;
+  if (!attachments.imageLimits.mediaTypes.includes(mediaType)) return undefined;
   if (
     data.byteLength >
     Math.min(attachments.imageLimits.maxImageBytes, attachments.imageLimits.maxMessageImageBytes)
@@ -55,9 +111,9 @@ export async function trySaveScreenshot(
   }
   if (!(await isImageCapableRoute(ctx, exec))) return undefined;
   try {
-    return await attachments.saveImage({ data, mediaType: "image/png", name });
+    return await attachments.saveImage({ data, mediaType, name });
   } catch {
-    // A store hiccup must not fail the tool call; the PNG path still works.
+    // A store hiccup must not fail the tool call; the path-only form still works.
     return undefined;
   }
 }