diff --git a/packages/server/src/server/bootstrap.ts b/packages/server/src/server/bootstrap.ts
index 5c1d0d1404..0ec792e871 100644
--- a/packages/server/src/server/bootstrap.ts
+++ b/packages/server/src/server/bootstrap.ts
@@ -126,6 +126,7 @@ import { createPaseoWorktree as createRegisteredPaseoWorktree } from "./paseo-wo
 import { createWorkspaceProvisioningService } from "./session/workspace-provisioning/workspace-provisioning-service.js";
 import { createPaseoWorktreeWorkflow } from "./worktree-session.js";
 import { DownloadTokenStore } from "./file-download/token-store.js";
+import { formatAttachmentContentDisposition } from "./file-download/content-disposition.js";
 import type { OpenAiSpeechProviderConfig } from "./speech/providers/openai/config.js";
 import type { LocalSpeechProviderConfig } from "./speech/providers/local/config.js";
 import type { RequestedSpeechProviders } from "./speech/speech-types.js";
@@ -836,8 +837,8 @@ export async function createPaseoDaemon(
         return;
       }
 
-      res.attachment(entry.fileName);
       res.setHeader("Content-Type", entry.mimeType);
+      res.setHeader("Content-Disposition", formatAttachmentContentDisposition(entry.fileName));
       res.setHeader("Content-Length", fileStats.size.toString());
 
       const stream = fileHandle.createReadStream();
diff --git a/packages/server/src/server/daemon-e2e/file-download.e2e.test.ts b/packages/server/src/server/daemon-e2e/file-download.e2e.test.ts
index 685d056a27..ac138a8c01 100644
--- a/packages/server/src/server/daemon-e2e/file-download.e2e.test.ts
+++ b/packages/server/src/server/daemon-e2e/file-download.e2e.test.ts
@@ -26,6 +26,10 @@ describe("daemon E2E", () => {
   describe("file download tokens", () => {
     test.each([
       { fileName: "download.txt", disposition: 'attachment; filename="download.txt"' },
+      {
+        fileName: "café.txt",
+        disposition: "attachment; filename=\"caf?.txt\"; filename*=UTF-8''caf%C3%A9.txt",
+      },
       {
         fileName: "中文报告 (最终版).txt",
         disposition:
diff --git a/packages/server/src/server/file-download/content-disposition.test.ts b/packages/server/src/server/file-download/content-disposition.test.ts
new file mode 100644
index 0000000000..46748c21ef
--- /dev/null
+++ b/packages/server/src/server/file-download/content-disposition.test.ts
@@ -0,0 +1,42 @@
+import { describe, expect, test } from "vitest";
+import { formatAttachmentContentDisposition } from "./content-disposition.js";
+
+describe("formatAttachmentContentDisposition", () => {
+  test.each([
+    { fileName: "report.txt", disposition: 'attachment; filename="report.txt"' },
+    {
+      fileName: "café.txt",
+      disposition: "attachment; filename=\"caf?.txt\"; filename*=UTF-8''caf%C3%A9.txt",
+    },
+    {
+      fileName: "Résumé Ångström.pdf",
+      disposition:
+        "attachment; filename=\"R?sum? ?ngstr?m.pdf\"; filename*=UTF-8''R%C3%A9sum%C3%A9%20%C3%85ngstr%C3%B6m.pdf",
+    },
+    {
+      fileName: "中文报告 (最终版).txt",
+      disposition:
+        "attachment; filename=\"???? (???).txt\"; filename*=UTF-8''%E4%B8%AD%E6%96%87%E6%8A%A5%E5%91%8A%20%28%E6%9C%80%E7%BB%88%E7%89%88%29.txt",
+    },
+    {
+      fileName: "report-📄.txt",
+      disposition:
+        "attachment; filename=\"report-??.txt\"; filename*=UTF-8''report-%F0%9F%93%84.txt",
+    },
+    {
+      fileName: 'say "hi" \\ bye.txt',
+      disposition: 'attachment; filename="say \\"hi\\" \\\\ bye.txt"',
+    },
+    {
+      fileName: "100%25 done.txt",
+      disposition: "attachment; filename=\"100%25 done.txt\"; filename*=UTF-8''100%2525%20done.txt",
+    },
+    {
+      fileName: "l'été (1)*.txt",
+      disposition:
+        "attachment; filename=\"l'?t? (1)*.txt\"; filename*=UTF-8''l%27%C3%A9t%C3%A9%20%281%29%2A.txt",
+    },
+  ])("$fileName", ({ fileName, disposition }) => {
+    expect(formatAttachmentContentDisposition(fileName)).toBe(disposition);
+  });
+});
diff --git a/packages/server/src/server/file-download/content-disposition.ts b/packages/server/src/server/file-download/content-disposition.ts
new file mode 100644
index 0000000000..1ad7997b7c
--- /dev/null
+++ b/packages/server/src/server/file-download/content-disposition.ts
@@ -0,0 +1,32 @@
+const NON_PRINTABLE_ASCII = /[^\x20-\x7e]/g;
+const PERCENT_ESCAPE = /%[0-9A-Fa-f]{2}/;
+const QUOTED_STRING_SPECIALS = /[\\"]/g;
+const NON_ATTR_CHARS_LEFT_BY_URI_ENCODING = /['()*]/g;
+
+/**
+ * Content-Disposition for a file download (RFC 6266).
+ *
+ * Any name that is not plain ASCII also gets `filename*=UTF-8''…`, because
+ * browsers do not read raw Latin-1 bytes in `filename` and fall back to a name
+ * taken from the URL. A name with a percent escape gets it too, so browsers do
+ * not decode the escape in `filename`.
+ */
+export function formatAttachmentContentDisposition(fileName: string): string {
+  const asciiFileName = fileName.replace(NON_PRINTABLE_ASCII, "?");
+  const disposition = `attachment; filename=${quote(asciiFileName)}`;
+  if (asciiFileName === fileName && !PERCENT_ESCAPE.test(fileName)) {
+    return disposition;
+  }
+  return `${disposition}; filename*=UTF-8''${encodeExtendedValue(fileName)}`;
+}
+
+function quote(value: string): string {
+  return `"${value.replace(QUOTED_STRING_SPECIALS, "\\$&")}"`;
+}
+
+function encodeExtendedValue(value: string): string {
+  return encodeURIComponent(value).replace(
+    NON_ATTR_CHARS_LEFT_BY_URI_ENCODING,
+    (char) => `%${char.charCodeAt(0).toString(16).toUpperCase()}`,
+  );
+}