refactor to allow more than file types on multipart upload

done for [AGIOS-131](https://issues.jboss.org/browse/AGIOS-131)

(Note that old behaviour of passing directly the nsurl can still be used but is deprecated in favour of the new approach.)

client project has been updated and exercises the API can be found here:
https://github.com/cvasilak/MultiPartIOSDemo/blob/master/MultiPartIOSDemo/Classes/Controllers/AGPViewController.m#L16