FileBasedHttpResponder.cs

// Redirects requests to a given file
class FileBasedHttpResponder : HttpClientHandler
{
    private readonly FileInfo _targetFileInfo;

    public FileBasedHttpResponder(string targetPath)
    {
        _targetFileInfo = new FileInfo(targetPath);
    }

    protected override Task<HttpResponseMessage> SendAsync(
        HttpRequestMessage request,
        CancellationToken cancellationToken)
    {
        HttpResponseMessage response;
        if (_targetFileInfo.Exists)
        {
            response = new HttpResponseMessage(HttpStatusCode.OK)
            {
                RequestMessage = request,
                Content = new StreamContent(_targetFileInfo.OpenRead())
            };
        }
        else
        {
            response = new HttpResponseMessage(HttpStatusCode.NotFound);
        }

        return Task.FromResult(response);
    }
}