Update April 4, 2016
As of release 6 of the app the dropdown for content type header back to the app. Read more in this blog post.
I received a lot of questions about missing drop-down with predefined encoding values. I feel I owe you an explanation.
Lets begin of what it was. Encoding for payload is nothing more than value for Content-Type header. So when you selected one of predefined encodings (or rather mime types) the app added Content-Type header to the headers list before submission.
Earlier, the drop down list contained a list of predefined values for the Content-Type header. In fact this was only a shortcut for adding a header to the headers list. We all love shortcuts but..