My basic requirement is proplist actually and I guess I can write a custom extractor for application/erlangproplist.
Just out of curiosity, as Riak is written in Erlang, is there any Erlang term extractor already available out there?
It's going to be very complex extractor I suppose since Erlang term can be in many forms like tuple, proplist etc.
No, there is no such extractor embedded in Riak. You can see all available extractors here. You will have to build your own. Chances are it would resemble yz_json_extractor a great deal.
If you need an extractor for a specific proplist format with some custom extraction logic you can find my old attempt here (gist.github.com/Vorticity-Flux/e0b52cb678ef4f997e06).