diff --git a/tests/web/attachments.test.ts b/tests/web/attachments.test.ts
new file mode 100644
index 00000000..3ecc84c6
--- /dev/null
+++ b/tests/web/attachments.test.ts
@@ -0,0 +1,21 @@
+import assert from "node:assert/strict";
+import test from "node:test";
+import { validateWebAttachments } from "../../web/protocol/attachments.ts";
+
+test("attachment validation accepts bounded supported files", () => {
+  assert.deepEqual(validateWebAttachments([
+    { name: "notes.md", mime: "text/markdown", size: 100 },
+    { name: "shot.png", mime: "image/png", size: 200 },
+  ]), { ok: true });
+});
+
+test("attachment validation rejects traversal, unsupported types, and oversized totals", () => {
+  assert.equal(validateWebAttachments([{ name: "../secret", mime: "text/plain", size: 1 }]).ok, false);
+  assert.equal(validateWebAttachments([{ name: "x.bin", mime: "application/octet-stream", size: 1 }]).ok, false);
+  assert.equal(validateWebAttachments([
+    { name: "a.txt", mime: "text/plain", size: 2 * 1024 * 1024 },
+    { name: "b.txt", mime: "text/plain", size: 2 * 1024 * 1024 },
+    { name: "c.txt", mime: "text/plain", size: 2 * 1024 * 1024 },
+    { name: "d.txt", mime: "text/plain", size: 2 * 1024 * 1024 + 1 },
+  ]).ok, false);
+});
diff --git a/web/protocol/attachments.ts b/web/protocol/attachments.ts
new file mode 100644
index 00000000..06b06af9
--- /dev/null
+++ b/web/protocol/attachments.ts
@@ -0,0 +1,42 @@
+export const WEB_MAX_ATTACHMENTS = 8;
+export const WEB_MAX_ATTACHMENT_BYTES = 2 * 1024 * 1024;
+export const WEB_MAX_ATTACHMENT_TOTAL_BYTES = 8 * 1024 * 1024;
+
+const SAFE_FILENAME = /^(?!\.\.?(?:$|\.))[\w .()\[\]-]{1,120}$/u;
+const SUPPORTED_MIME = new Set([
+  "text/plain",
+  "text/markdown",
+  "application/json",
+  "image/png",
+  "image/jpeg",
+  "image/webp",
+]);
+
+export interface WebAttachmentInput {
+  readonly name: string;
+  readonly mime: string;
+  readonly size: number;
+}
+
+export function validateWebAttachments(attachments: readonly WebAttachmentInput[]) {
+  if (attachments.length > WEB_MAX_ATTACHMENTS) {
+    return { ok: false as const, error: `at most ${WEB_MAX_ATTACHMENTS} attachments are allowed` };
+  }
+  let total = 0;
+  for (const attachment of attachments) {
+    if (!SAFE_FILENAME.test(attachment.name) || attachment.name.includes("..")) {
+      return { ok: false as const, error: `invalid attachment name: ${attachment.name}` };
+    }
+    if (!SUPPORTED_MIME.has(attachment.mime)) {
+      return { ok: false as const, error: `unsupported attachment type: ${attachment.mime}` };
+    }
+    if (!Number.isSafeInteger(attachment.size) || attachment.size < 0 || attachment.size > WEB_MAX_ATTACHMENT_BYTES) {
+      return { ok: false as const, error: `attachment exceeds ${WEB_MAX_ATTACHMENT_BYTES} byte limit` };
+    }
+    total += attachment.size;
+    if (total > WEB_MAX_ATTACHMENT_TOTAL_BYTES) {
+      return { ok: false as const, error: `attachments exceed ${WEB_MAX_ATTACHMENT_TOTAL_BYTES} byte total` };
+    }
+  }
+  return { ok: true as const };
+}