API Content Negotiation Designer

You are a standards-focused API architect implementing proper HTTP content negotiation.

## CONTEXT
I need to support multiple response formats in my API.

**Requirements:**
- Formats Needed: {{FORMATS}}
- Client Types: {{CLIENTS}}
- Default Format: {{DEFAULT}}
- Legacy Support: {{LEGACY}}

## TASK
Design content negotiation:

### 1. ACCEPT HEADER
```
GET /api/users
Accept: application/json

GET /api/users
Accept: text/csv

GET /api/users
Accept: application/xml
```

### 2. QUALITY VALUES
```
Accept: application/json;q=1.0, application/xml;q=0.8, */*;q=0.1
```

### 3. FORMAT EXTENSION (Alternative)
```
GET /api/users.json
GET /api/users.csv
GET /api/users.xml
```

### 4. RESPONSE HEADERS
```
Content-Type: application/json; charset=utf-8
Vary: Accept
```

### 5. IMPLEMENTATION
```typescript
const formatters = {
  'application/json': (data) => JSON.stringify(data),
  'text/csv': (data) => convertToCsv(data),
  'application/xml': (data) => convertToXml(data),
};

function negotiate(req, data) {
  const accept = req.headers.accept;
  const format = selectFormat(accept, Object.keys(formatters));
  return {
    contentType: format,
    body: formatters[format](data)
  };
}
```

### 6. 406 NOT ACCEPTABLE
```json
{
  "error": "Not Acceptable",
  "message": "Supported formats: application/json, text/csv",
  "supportedTypes": ["application/json", "text/csv"]
}
```