TTS audio fails to play in browser due to Content-Type mismatch (WAV data served as audio/mpeg)

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### Dify version

1.15.0

### Cloud or Self Hosted

Self Hosted (Docker)

### Steps to reproduce

1.Configure TTS in ChatFlow using Tongyi's qwen3-tts-flash model.

2.Generate a conversation and click the audio playback button.

3.Observe that no sound is played.

### ✔️ Expected Behavior

Audio plays normally in the browser.

### ❌ Actual Behavior

TTS audio fails to play in the browser. The browser console shows the following error:

**Chrome**:
```
Uncaught (in promise) NotSupportedError: Failed to load because no supported source was found.
```

**Edge**:
```
Uncaught (in promise) NotSupportedError: Failed to load because no supported source was found.
```

**Firefox**:
```
Uncaught DOMException: MediaSource.addSourceBuffer: Type not supported in MediaSource
```

Upon inspecting the API response, I found:
- The actual audio data returned is **WAV format** (starts with `RIFF....WAVE`)
- But the HTTP response `Content-Type` is set to **`audio/mpeg`**

This is exactly the same root cause identified in [#35880](https://github.com/langgenius/dify/issues/35880) by @euxx:

> "The Tongyi TTS implementation does not specify an output format and returns the downloaded audio bytes unchanged. The actual response is WAV data (RIFF....WAVE, PCM 24 kHz), while Dify responds with Content-Type: audio/mpeg."

Although [#35901](https://github.com/langgenius/dify/pull/35901) was merged to fix AudioContext issues, the **audio format mismatch** between the actual WAV data and the `audio/mpeg` Content-Type header persists in v1.15.0.

Start at the Tongyi qwen3-tts-flash response path used by ChatFlow and inspect where the API response Content-Type is assigned; confirm that the returned bytes begin with RIFF/WAVE. Done means browser playback works in Chrome, Edge, and Firefox without the reported unsupported-source errors.

Written by the indexing model from the issue text.