diff --git a/src/server/files/kv-transit-files.test.ts b/src/server/files/kv-transit-files.test.ts
index 3c3597e5..4852e252 100644
--- a/src/server/files/kv-transit-files.test.ts
+++ b/src/server/files/kv-transit-files.test.ts
@@ -138,6 +138,19 @@ describe("KVTransitFileService", () => {
     expect(response.headers.get("content-disposition")).toBe('attachment; filename="a_b_c_d_e.txt"');
   });
 
+  it("serves a file whose name is outside Latin-1", async () => {
+    const namespace = new MemoryKVNamespace();
+    const service = createService(namespace);
+    const upload = await service.create(new File(["ok"], "发票.pdf", { type: "application/pdf" }));
+
+    const response = await service.response(upload.fileId);
+
+    expect(response.status).toBe(200);
+    expect(response.headers.get("content-disposition")).toBe(
+      "attachment; filename=\"__.pdf\"; filename*=UTF-8''%E5%8F%91%E7%A5%A8.pdf",
+    );
+  });
+
   it("rejects non-integer, non-positive, or non-finite ttl/maxBytes at construction", () => {
     // NaN maxBytes would otherwise slip past Math.min and disable the size check entirely.
     for (const bad of [Number.NaN, Number.POSITIVE_INFINITY, 0, -1, 1.5]) {
diff --git a/src/server/files/kv-transit-files.ts b/src/server/files/kv-transit-files.ts
index 7a4b7d5d..eed92ee5 100644
--- a/src/server/files/kv-transit-files.ts
+++ b/src/server/files/kv-transit-files.ts
@@ -2,7 +2,7 @@ import type { KVNamespaceBinding } from "../cloudflare/cloudflare-bindings.ts";
 import type { ITransitFileService, TransitFileRead, TransitFileUpload } from "./transit-file-store.ts";
 
 import { extname } from "node:path";
-import { contentTypeFromFileId, TransitFileError } from "./transit-file-store.ts";
+import { contentDispositionForFileName, contentTypeFromFileId, TransitFileError } from "./transit-file-store.ts";
 
 // Workers KV rejects an `expirationTtl` below 60 seconds.
 const KV_MIN_TTL_SECONDS = 60;
@@ -83,7 +83,7 @@ export class KVTransitFileService implements ITransitFileService {
       headers: {
         "content-length": String(metadata.sizeBytes),
         "content-type": metadata.mimeType,
-        "content-disposition": `attachment; filename="${escapeHeaderValue(metadata.name)}"`,
+        "content-disposition": contentDispositionForFileName(metadata.name),
       },
     });
   }
@@ -151,9 +151,6 @@ function positiveInteger(value: number, field: string): number {
   }
   return value;
 }
-function escapeHeaderValue(value: string): string {
-  return value.replace(/["\\\r\n]/g, "_");
-}
 function safeExtension(name: string): string {
   const extension = extname(name).toLowerCase();
   return /^\.[a-z0-9]{1,16}$/.test(extension) ? extension : "";
diff --git a/src/server/files/r2-transit-files.ts b/src/server/files/r2-transit-files.ts
index c830e69c..4d4efaa0 100644
--- a/src/server/files/r2-transit-files.ts
+++ b/src/server/files/r2-transit-files.ts
@@ -2,7 +2,7 @@ import type { R2BucketBinding, R2ObjectBinding } from "../cloudflare/cloudflare-
 import type { ITransitFileService, TransitFileRead, TransitFileUpload } from "./transit-file-store.ts";
 
 import { extname } from "node:path";
-import { contentTypeFromFileId, TransitFileError } from "./transit-file-store.ts";
+import { contentDispositionForFileName, contentTypeFromFileId, TransitFileError } from "./transit-file-store.ts";
 
 export interface R2TransitFileOptions {
   bucket: R2BucketBinding;
@@ -71,7 +71,7 @@ export class R2TransitFileService implements ITransitFileService {
       headers: {
         "content-length": String(metadata.sizeBytes),
         "content-type": metadata.mimeType,
-        "content-disposition": `attachment; filename="${escapeHeaderValue(metadata.name)}"`,
+        "content-disposition": contentDispositionForFileName(metadata.name),
       },
     });
   }
@@ -151,10 +151,6 @@ function assertSafeFileId(fileId: string): void {
   }
 }
 
-function escapeHeaderValue(value: string): string {
-  return value.replace(/["\\\r\n]/g, "_");
-}
-
 function safeExtension(name: string): string {
   const extension = extname(name).toLowerCase();
   return /^\.[a-z0-9]{1,16}$/.test(extension) ? extension : "";
diff --git a/src/server/files/transit-file-store.test.ts b/src/server/files/transit-file-store.test.ts
new file mode 100644
index 00000000..5169dfbf
--- /dev/null
+++ b/src/server/files/transit-file-store.test.ts
@@ -0,0 +1,46 @@
+import { describe, expect, it } from "vitest";
+import { contentDispositionForFileName, createTransitFileResponse } from "./transit-file-store.ts";
+
+describe("contentDispositionForFileName", () => {
+  it("leaves an ASCII file name in the plain filename parameter", () => {
+    expect(contentDispositionForFileName("report.TXT")).toBe('attachment; filename="report.TXT"');
+  });
+
+  it("escapes quotes, backslashes, and control bytes without adding an encoded parameter", () => {
+    expect(contentDispositionForFileName('a"b\\c\rd\ne.txt')).toBe('attachment; filename="a_b_c_d_e.txt"');
+  });
+
+  it("carries a non-ASCII file name in filename* with an ASCII fallback", () => {
+    expect(contentDispositionForFileName("发票.pdf")).toBe(
+      "attachment; filename=\"__.pdf\"; filename*=UTF-8''%E5%8F%91%E7%A5%A8.pdf",
+    );
+  });
+
+  it("encodes characters that are percent-encoded but not attr-char", () => {
+    expect(contentDispositionForFileName("i'nvoice(1)*.pdf—x")).toBe(
+      "attachment; filename=\"i'nvoice(1)*.pdf_x\"; filename*=UTF-8''i%27nvoice%281%29%2A.pdf%E2%80%94x",
+    );
+  });
+
+  it("counts an astral character as a single replacement", () => {
+    expect(contentDispositionForFileName("chart\u{1f4ca}.pdf")).toBe(
+      "attachment; filename=\"chart_.pdf\"; filename*=UTF-8''chart%F0%9F%93%8A.pdf",
+    );
+  });
+});
+
+describe("createTransitFileResponse", () => {
+  it("builds a response for a file whose name is outside Latin-1", async () => {
+    const response = createTransitFileResponse({
+      file: new File(["hello"], "发票.pdf", { type: "application/pdf" }),
+      sizeBytes: 5,
+      name: "发票.pdf",
+      mimeType: "application/pdf",
+    });
+
+    expect(response.headers.get("content-disposition")).toBe(
+      "attachment; filename=\"__.pdf\"; filename*=UTF-8''%E5%8F%91%E7%A5%A8.pdf",
+    );
+    expect(await response.text()).toBe("hello");
+  });
+});
diff --git a/src/server/files/transit-file-store.ts b/src/server/files/transit-file-store.ts
index 20ec985e..ad55a539 100644
--- a/src/server/files/transit-file-store.ts
+++ b/src/server/files/transit-file-store.ts
@@ -38,11 +38,36 @@ export function createTransitFileResponse(file: TransitFileRead): Response {
     headers: {
       "content-length": String(file.sizeBytes),
       "content-type": file.mimeType,
-      "content-disposition": `attachment; filename="${escapeHeaderValue(file.name)}"`,
+      "content-disposition": contentDispositionForFileName(file.name),
     },
   });
 }
 
+/**
+ * Build the `content-disposition` value for a transit file download.
+ *
+ * Header values are ByteStrings, so a name holding a character above U+00FF
+ * throws while the response is constructed and the download fails. Such names
+ * travel in the RFC 6266 `filename*` parameter, and `filename` keeps an
+ * ASCII-only form for clients that do not read `filename*`.
+ */
+export function contentDispositionForFileName(name: string): string {
+  const asciiName = name.replace(/[^\u0020-\u007e]/gu, "_").replace(/["\\]/g, "_");
+  if (!/[\u0080-\u{10ffff}]/u.test(name)) {
+    return `attachment; filename="${asciiName}"`;
+  }
+
+  return `attachment; filename="${asciiName}"; filename*=UTF-8''${encodeExtendedValue(name)}`;
+}
+
+/** Percent-encode a file name as an RFC 8187 `ext-value`, which allows fewer literals than a URI component. */
+function encodeExtendedValue(name: string): string {
+  return encodeURIComponent(name).replace(
+    /['()*]/g,
+    (character) => `%${character.charCodeAt(0).toString(16).toUpperCase()}`,
+  );
+}
+
 export function contentTypeFromFileId(fileId: string): string {
   const dotIndex = fileId.lastIndexOf(".");
   const extension = dotIndex === -1 ? "" : fileId.slice(dotIndex).toLowerCase();
@@ -94,7 +119,3 @@ export function contentTypeFromFileId(fileId: string): string {
       return "application/octet-stream";
   }
 }
-
-function escapeHeaderValue(value: string): string {
-  return value.replace(/["\\\r\n]/g, "_");
-}
diff --git a/src/server/files/transit-files.ts b/src/server/files/transit-files.ts
index bed8c9d9..18878d5d 100644
--- a/src/server/files/transit-files.ts
+++ b/src/server/files/transit-files.ts
@@ -7,7 +7,7 @@ import { mkdir, readFile, readdir, rename, stat, unlink, writeFile } from "node:
 import { extname, join } from "node:path";
 import { Readable } from "node:stream";
 import { finished } from "node:stream/promises";
-import { contentTypeFromFileId, TransitFileError } from "./transit-file-store.ts";
+import { contentDispositionForFileName, contentTypeFromFileId, TransitFileError } from "./transit-file-store.ts";
 
 export interface TransitFileOptions {
   rootDir: string;
@@ -97,7 +97,7 @@ export class TransitFileService implements ITransitFileService {
       headers: {
         "content-length": String(stats.size),
         "content-type": metadata.mimeType,
-        "content-disposition": `attachment; filename="${escapeHeaderValue(metadata.name)}"`,
+        "content-disposition": contentDispositionForFileName(metadata.name),
       },
     });
   }
@@ -218,7 +218,3 @@ function normalizeMetadata(
     typeof input.mimeType === "string" && input.mimeType.trim() ? input.mimeType.trim() : fallback.mimeType;
   return { name, mimeType };
 }
-
-function escapeHeaderValue(value: string): string {
-  return value.replace(/["\\\r\n]/g, "_");
-}