Hello @cheper,
Unfortunately I don’t have the list of possible reqAttribute. I guess this is an HTTP request concept, abstracted in Java and that can be anything (even though there are some standard attributes).
I quickly searched for java request.getAttribute but did not find anything super clear.
This page seems one of the most interesting, without any guarantees…
And indeed adding %requestContent in the log formatting string is nice since it displays all parameters of a POST … but it also shows user password during authentication requests 
Olivier