diff --git a/src/backlog.ts b/src/backlog.ts
index 2c723be..fbd2435 100644
--- a/src/backlog.ts
+++ b/src/backlog.ts
@@ -3,6 +3,43 @@ import * as Entity from "./entity";
 import Request from "./request";
 import type { Fetch } from "./types";
 
+/**
+ * Extracts the filename from a `Content-Disposition` header, or `""` when it
+ * carries none.
+ *
+ * Per RFC 6266 the `filename*` extended notation wins over plain `filename`;
+ * its `<charset>'<language>'` prefix is dropped and the rest percent-decoded.
+ * The value is the server's, so sanitise it before using it as a path.
+ */
+const parseContentDispositionFilename = (disposition: string | null): string => {
+  if (!disposition) {
+    return "";
+  }
+
+  const extended = /(?:^|;)\s*filename\*\s*=\s*([^;]+)/i.exec(disposition);
+  if (extended) {
+    // Servers send a quoted ext-value even though RFC 5987 forbids it.
+    const value = extended[1].trim().replace(/^"(.*)"$/, "$1");
+    const encoded = /^[^']*'[^']*'(.*)$/.exec(value);
+    if (encoded) {
+      try {
+        return decodeURIComponent(encoded[1]);
+      } catch {
+        return encoded[1];
+      }
+    }
+  }
+
+  // Before the unquoted form: a quoted value may contain the `;` that ends one.
+  const quoted = /(?:^|;)\s*filename\s*=\s*"((?:[^"\\]|\\.)*)"/i.exec(disposition);
+  if (quoted) {
+    return quoted[1].replace(/\\(.)/g, "$1");
+  }
+
+  const plain = /(?:^|;)\s*filename\s*=\s*([^;]*)/i.exec(disposition);
+  return plain ? plain[1].trim() : "";
+};
+
 export default class Backlog extends Request {
   constructor(configure: {
     host: string;
@@ -1408,12 +1445,10 @@ export default class Backlog extends Request {
           blob: () => response.blob(),
         });
       } else {
-        const disposition = response.headers.get("Content-Disposition");
-        const filename = disposition ? disposition.substring(disposition.indexOf("''") + 2) : "";
         resolve({
           body: (<any>response).body,
           url: response.url,
-          filename: filename,
+          filename: parseContentDispositionFilename(response.headers.get("Content-Disposition")),
         });
       }
     });
diff --git a/test/test.ts b/test/test.ts
index 6ff4bf3..0758ef6 100644
--- a/test/test.ts
+++ b/test/test.ts
@@ -494,6 +494,80 @@ describe("Backlog API", () => {
     expect(data).toHaveProperty("filename", "test.png");
   });
 
+  // Measured against a live Backlog space: it always uses the extended notation
+  // and percent-encodes anything that needs it, so `%20`, `%3B` and `%2530` are
+  // what an ordinary filename arrives as.
+  it.each([
+    ["attachment; filename*=UTF-8''shot.png", "shot.png"],
+    ["attachment; filename*=UTF-8''%E5%9B%B3%E9%9D%A2.png", "図面.png"],
+    ["attachment; filename*=UTF-8''a%20b.csv", "a b.csv"],
+    ["attachment; filename*=UTF-8''q1%3Bsummary.csv", "q1;summary.csv"],
+    ["attachment; filename*=UTF-8''20%2530report.csv", "20%30report.csv"],
+  ])("should read the filename Backlog sends in %j", async (disposition, expected) => {
+    const issueIdOrKey = "BLG-1";
+    const attachmentId = 3;
+    mockRequest({
+      method: "GET",
+      path: `/api/v2/issues/${issueIdOrKey}/attachments/${attachmentId}`,
+      reqHeaders: { "Backlog-API-Key": apiKey },
+      status: 200,
+      data: "dummy",
+      headers: { "Content-Disposition": disposition },
+      times: 1,
+    });
+    const data = await backlog.getIssueAttachment(issueIdOrKey, attachmentId);
+    expect(data).toHaveProperty("filename", expected);
+  });
+
+  // Shapes RFC 6266 permits that Backlog does not send, so the client does not
+  // depend on one server's habits.
+  it.each([
+    ['attachment; filename="report.png"', "report.png"],
+    ["attachment; filename=report.png", "report.png"],
+    ["attachment; filename*=UTF-8''%E5%9B%B3%E9%9D%A2.png", "図面.png"],
+    // RFC 6266: `filename*` wins when a header carries both.
+    ["attachment; filename=\"fallback.png\"; filename*=UTF-8''%E5%9B%B3.png", "図.png"],
+    // A quoted value may contain the delimiter.
+    ['attachment; filename="quarter;summary.pdf"', "quarter;summary.pdf"],
+    ['attachment; filename="20%30report.pdf"', "20%30report.pdf"],
+    // RFC 5987 does not allow a quoted ext-value, but servers send one.
+    ["attachment; filename*=\"UTF-8''%E5%9B%B3.png\"", "図.png"],
+    ['attachment; filename="  spaced.png  "', "  spaced.png  "],
+    ["attachment; filename=  spaced.png  ", "spaced.png"],
+    ['attachment; filename="a\\"b.png"', 'a"b.png'],
+    ["attachment;filename=nospace.png", "nospace.png"],
+    ["attachment", ""],
+  ])("should read the filename from %j", async (disposition, expected) => {
+    const issueIdOrKey = "BLG-1";
+    const attachmentId = 1;
+    mockRequest({
+      method: "GET",
+      path: `/api/v2/issues/${issueIdOrKey}/attachments/${attachmentId}`,
+      reqHeaders: { "Backlog-API-Key": apiKey },
+      status: 200,
+      data: "dummy",
+      headers: { "Content-Disposition": disposition },
+      times: 1,
+    });
+    const data = await backlog.getIssueAttachment(issueIdOrKey, attachmentId);
+    expect(data).toHaveProperty("filename", expected);
+  });
+
+  it("should return an empty filename when the header is absent.", async () => {
+    const issueIdOrKey = "BLG-1";
+    const attachmentId = 2;
+    mockRequest({
+      method: "GET",
+      path: `/api/v2/issues/${issueIdOrKey}/attachments/${attachmentId}`,
+      reqHeaders: { "Backlog-API-Key": apiKey },
+      status: 200,
+      data: "dummy",
+      times: 1,
+    });
+    const data = await backlog.getIssueAttachment(issueIdOrKey, attachmentId);
+    expect(data).toHaveProperty("filename", "");
+  });
+
   it("should add a document.", async () => {
     mockRequest({
       method: "POST",