diff --git a/client/src/lib/luxsinApi.ts b/client/src/lib/luxsinApi.ts
index ce31dfb..0a6d869 100644
--- a/client/src/lib/luxsinApi.ts
+++ b/client/src/lib/luxsinApi.ts
@@ -423,7 +423,7 @@ export class LuxsinAPI {
       ? `/fir-device/${host}/${side}`
       : `${this.baseUrl}/dev/info.cgi?action=fir&channel=${side}`;
     const response = await this.timedFetch(url, {
-      method: 'POST', headers: { 'Content-Type': 'audio/wav' }, body: encodeDeviceFirWav(data),
+      method: 'POST', body: encodeDeviceFirWav(data),
     });
     if (!response.ok) {
       let reason = '';
diff --git a/scripts/fir-regression.test.mjs b/scripts/fir-regression.test.mjs
index b440336..899ad91 100644
--- a/scripts/fir-regression.test.mjs
+++ b/scripts/fir-regression.test.mjs
@@ -29,7 +29,7 @@ test('actual API sends only raw left/right binary, never FIR settings',async()=>
  const l=encodeFir(coefficients()),r=encodeFir(Array(8191).fill(-0.125));
  await applyFir(new LuxsinAPI('192.168.9.127'),l,r);
  assert.deepEqual(requests.map(r=>r.url),['http://192.168.9.127/dev/info.cgi?action=fir&channel=left','http://192.168.9.127/dev/info.cgi?action=fir&channel=right']);
- for (const [index,b] of [[0,l],[1,r]]) {assert.equal(requests[index].init.method,'POST');assert.equal(requests[index].init.headers['Content-Type'],'audio/wav');assert.deepEqual(new Uint8Array(decodeFirWav(requests[index].init.body)),new Uint8Array(decodeFirWav(encodeDeviceFirWav(b))));}
+ for (const [index,b] of [[0,l],[1,r]]) {assert.equal(requests[index].init.method,'POST');assert.equal(new Headers(requests[index].init.headers).has('Content-Type'),false);assert.deepEqual(new Uint8Array(decodeFirWav(requests[index].init.body)),new Uint8Array(decodeFirWav(encodeDeviceFirWav(b))));}
 });
 test('manual 8192-tap bytes preserved, enable/disable setting contract',async()=>{
  const requests=[];globalThis.fetch=async(url,init)=>{requests.push({url,init});return new Response('OK');};
@@ -149,7 +149,7 @@ test('legacy 44.1 kHz room becomes device-compatible 48 kHz WAV', async () => {
  await applyFir(new LuxsinAPI('192.168.9.133'), left, right);
  assert.equal(requests.length, 2);
  for (const req of requests) {
-   assert.equal(req.headers['Content-Type'], 'audio/wav');
+   assert.equal(new Headers(req.headers).has('Content-Type'), false);
    assert.equal(new DataView(req.body).getUint32(24, true), 48000);
    assert.equal(req.body.byteLength, 32826);
    const samples = new DataView(decodeFirWav(req.body));