I don't remember why I made this change a year ago. I created #141 for that, but explanations are definitely too short :)
Read #169 also. Another user tried to provide a better upload.

After this change, no one has reported any upload hangs. Neither OOME. I think that was a good change. But for an old version of Play framework now.
Maybe Enumerator#fromStream(...) could perform better with Play 2.2.
But I have to test case to reproduce upload hangs on slow connections and so I'm not able to evaluation the quality of a such modification.

> if the reads block (not using fromStream) then it's the Servlet 
> Thread which is blocked, and with fromStream it's a thread on the supplied 
> ExecutionContext?

Yes, you're right.