diff --git a/packages/opencode/src/util/media.ts b/packages/opencode/src/util/media.ts
index 566ac843..580f3ad6 100644
--- a/packages/opencode/src/util/media.ts
+++ b/packages/opencode/src/util/media.ts
@@ -12,15 +12,25 @@ export function isImageAttachment(mime: string) {
   return mime.startsWith("image/") && mime !== "image/svg+xml" && mime !== "image/vnd.fastbidsheet"
 }
 
-export function sniffAttachmentMime(bytes: Uint8Array, fallback: string) {
-  if (startsWith(bytes, [0x89, 0x50, 0x4e, 0x47, 0x0d, 0x0a, 0x1a, 0x0a])) return "image/png"
-  if (startsWith(bytes, [0xff, 0xd8, 0xff])) return "image/jpeg"
-  if (startsWith(bytes, [0x47, 0x49, 0x46, 0x38])) return "image/gif"
-  if (startsWith(bytes, [0x42, 0x4d])) return "image/bmp"
-  if (startsWith(bytes, [0x25, 0x50, 0x44, 0x46, 0x2d])) return "application/pdf"
-  if (startsWith(bytes, [0x52, 0x49, 0x46, 0x46]) && startsWith(bytes.subarray(8), [0x57, 0x45, 0x42, 0x50])) {
-    return "image/webp"
-  }
+interface MimeSignature {
+  mime: string
+  matches: (bytes: Uint8Array) => boolean
+}
 
-  return fallback
+const MIME_SIGNATURES: MimeSignature[] = [
+  { mime: "image/png", matches: (bytes) => startsWith(bytes, [0x89, 0x50, 0x4e, 0x47, 0x0d, 0x0a, 0x1a, 0x0a]) },
+  { mime: "image/jpeg", matches: (bytes) => startsWith(bytes, [0xff, 0xd8, 0xff]) },
+  { mime: "image/gif", matches: (bytes) => startsWith(bytes, [0x47, 0x49, 0x46, 0x38]) },
+  { mime: "image/bmp", matches: (bytes) => startsWith(bytes, [0x42, 0x4d]) },
+  { mime: "application/pdf", matches: (bytes) => startsWith(bytes, [0x25, 0x50, 0x44, 0x46, 0x2d]) },
+  {
+    mime: "image/webp",
+    matches: (bytes) =>
+      startsWith(bytes, [0x52, 0x49, 0x46, 0x46]) && startsWith(bytes.subarray(8), [0x57, 0x45, 0x42, 0x50]),
+  },
+]
+
+export function sniffAttachmentMime(bytes: Uint8Array, fallback: string) {
+  const signature = MIME_SIGNATURES.find((candidate) => candidate.matches(bytes))
+  return signature?.mime ?? fallback
 }
diff --git a/packages/opencode/test/util/media.test.ts b/packages/opencode/test/util/media.test.ts
new file mode 100644
index 00000000..8f3d4932
--- /dev/null
+++ b/packages/opencode/test/util/media.test.ts
@@ -0,0 +1,66 @@
+import { describe, expect, test } from "bun:test"
+import {
+  isImageAttachment,
+  isMedia,
+  isPdfAttachment,
+  sniffAttachmentMime,
+} from "../../src/util/media"
+
+describe("util.media", () => {
+  test("detects PNG from magic bytes", () => {
+    const bytes = new Uint8Array([0x89, 0x50, 0x4e, 0x47, 0x0d, 0x0a, 0x1a, 0x0a, 0x00, 0x00])
+    expect(sniffAttachmentMime(bytes, "application/octet-stream")).toBe("image/png")
+  })
+
+  test("detects JPEG from magic bytes", () => {
+    const bytes = new Uint8Array([0xff, 0xd8, 0xff, 0x00, 0x00])
+    expect(sniffAttachmentMime(bytes, "application/octet-stream")).toBe("image/jpeg")
+  })
+
+  test("detects GIF from magic bytes", () => {
+    const bytes = new Uint8Array([0x47, 0x49, 0x46, 0x38, 0x00, 0x00])
+    expect(sniffAttachmentMime(bytes, "application/octet-stream")).toBe("image/gif")
+  })
+
+  test("detects BMP from magic bytes", () => {
+    const bytes = new Uint8Array([0x42, 0x4d, 0x00, 0x00])
+    expect(sniffAttachmentMime(bytes, "application/octet-stream")).toBe("image/bmp")
+  })
+
+  test("detects PDF from magic bytes", () => {
+    const bytes = new Uint8Array([0x25, 0x50, 0x44, 0x46, 0x2d, 0x00])
+    expect(sniffAttachmentMime(bytes, "application/octet-stream")).toBe("application/pdf")
+  })
+
+  test("detects WEBP from RIFF + WEBP markers", () => {
+    const bytes = new Uint8Array([0x52, 0x49, 0x46, 0x46, 0x00, 0x00, 0x00, 0x00, 0x57, 0x45, 0x42, 0x50])
+    expect(sniffAttachmentMime(bytes, "application/octet-stream")).toBe("image/webp")
+  })
+
+  test("does not detect WEBP from RIFF alone without WEBP marker", () => {
+    const bytes = new Uint8Array([0x52, 0x49, 0x46, 0x46, 0x00, 0x00, 0x00, 0x00, 0xff, 0xff, 0xff, 0xff])
+    expect(sniffAttachmentMime(bytes, "application/octet-stream")).toBe("application/octet-stream")
+  })
+
+  test("returns fallback for unrecognized bytes", () => {
+    const bytes = new Uint8Array([0x00, 0x01, 0x02, 0x03])
+    expect(sniffAttachmentMime(bytes, "text/plain")).toBe("text/plain")
+  })
+
+  test("isPdfAttachment matches only application/pdf", () => {
+    expect(isPdfAttachment("application/pdf")).toBe(true)
+    expect(isPdfAttachment("image/png")).toBe(false)
+  })
+
+  test("isMedia matches images and pdf", () => {
+    expect(isMedia("image/png")).toBe(true)
+    expect(isMedia("application/pdf")).toBe(true)
+    expect(isMedia("text/plain")).toBe(false)
+  })
+
+  test("isImageAttachment excludes svg and fastbidsheet", () => {
+    expect(isImageAttachment("image/png")).toBe(true)
+    expect(isImageAttachment("image/svg+xml")).toBe(false)
+    expect(isImageAttachment("image/vnd.fastbidsheet")).toBe(false)
+  })
+})
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