file.cs

[Route("")]
public HttpResponseMessage GetResult()
{
    var result = File.ReadAllText(@"C:\data\SwissProt.xml");
    var response = new HttpResponseMessage(HttpStatusCode.OK)
    {   
        Content = new StringContent(result)
    };
    response.Content.Headers.ContentType = new MediaTypeHeaderValue("application/xml");
    return response;
}