Extend api to handle bot-user file exchange story

[mpustelnik]

Extend fluent api to allow writing easy test scenarios with files, eg:

public class FilesTests: BotTestBase<DemoBot>
{
    [Fact]
    public async Task UserSendsFile()
    {
        var story = this.Record
            .User.SendsFile(...)
            .Bot.Says("File received")
            .Rewind();

        await this.Play(story);
    }

    [Fact]
    public async Task BotSendsFile()
    {
        var story = this.Record
            .User.Says("Send me a file")
            .Bot.SendsFile(x => ...)
            .Rewind();

        await this.Play(story);
    }
}