controller.cs

public class LayoutController : ApiController
{
    [HttpGet]
    [Route("", Name = "Index")]
    public async Task<HttpResponseMessage> LayoutHtml()
    {
        var path = HttpContext.Current.IsDebuggingEnabled
                       ? HttpContext.Current.Server.MapPath("~/App/Views/Index.html")
                       : HttpContext.Current.Server.MapPath("~/App/View/Index_Release.html");
        var file = await File.OpenText(path).ReadToEndAsync();


        var responseMessage = this.Request.CreateResponse(HttpStatusCode.OK);
        var content = new StringContent(file);
        responseMessage.Content = content;
        responseMessage.Content.Headers.ContentType = new MediaTypeHeaderValue("text/html");

        return responseMessage;
    }
}