First a few caveats:

1.  There is no server side implementation / spec yet so the client "solution" should be rather pluggable.
2.  I've only looked at iOS as a sanity check on my ideas.

I've been working on some PoC's for multipart on Android.  After a few false starts here is the idea I've come up with to support multipart/from-data content types (ie uploading files).

Add a new configuration option to a Pipe's configuration: a request builder.

The request builder will be a callback/closure/class that consumes the data arguments from a pipes save request and produces the body of the request.  In Android it can be an interface which is implemented and in iOS it can be a block which is passed to AFHttpClient's multipartFormRequestWithMethod.  I'm not sure what the implications to JavaScript would be.

Specifically in Android the gson specific request code will be refactored into a GSONRequestBuilder, and we will write (and define the behavior of) a MultipartRequestBuilder.  Also PipeConfig.setGsonBuilder will become deprecated (a good move IMHO).

wdyt?

Summers