Why doesn't `Servant.Server.Internal.streamRouter` use `contentTypes`?

So, I have a service that is basically a dumb pipe streaming data between two other (legacy) services. Said data can be in different formats and I'd like to use the Accept header to choose between them, using the following type:

```
data MagicStream

instance Accept MagicStream where
contentTypes _ = ["application/json", "text/csv"]

instance MimeRender MagicStream ByteString where
mimeRender _ = ByteString.Lazy.fromStrict
```

Then my API would look like this:

```
type Api = Header "Accept" ActualFormat :> ... :> StreamGet NoFraming MagicStream ...
```

Sadly, this is currently not possible and I have to invent my own `X-Magic-Accept` header to work around `streamRouter`'s use of `contentType` (only the first value in the `contentTypes` list is accepted).

Is that a concious design choice? I wouldn't be surprised if I'm missing something here.