Tip 141 - Async write of multiple files to response

[njannink]

What would you like to know?

When trying the example of tip 141 on the latest version you will get the following error:

Synchronous operations are disallowed. Call WriteAsync or set AllowSynchronousIO to true instead.

The simple way is to set AllowSynchronousIO to true. But I wonder isn't there a way to do this asynchronous?