How to send files additional headers
### Discussed in https://github.com/HapticX/happyx/discussions/225

<div type='discussions-op-text'>

<sup>Originally posted by **derekdai** December 20, 2023</sup>
I have tried with
```nim
serve("localhost", 5000):
  "/":
    let headers = newCustomHeaders()
    headers["x"] = "y"
    return FileResponse("index.html", headers = headers)
```

It compiles without problem, but the `headers = header` part are discarded.
```nim
  of HttpMethod(1):                                                                            
    if `==`(urlPath, "/"):                                                                     
      let headers = newCustomHeaders()                                                         
      headers["x"] = "y"                                                                       
      await(answerFile(req, "index.html"))
```

No luck with `outHeaders`
```nim
serve("localhost", 5000):
  "/":
    outHeaders["x"] = "y"
    return FileResponse("index.html")
```

```nim
  of HttpMethod(1):                                                                            
    if `==`(urlPath, "/"):                                                                     
      var outHeaders = newCustomHeaders()                                                      
      outHeaders["x"] = "y"                                                                    
      await(answerFile(req, "index.html"))
```</div>