Improve testability of the attachment class.
Improve testability of the attachment class.

Improved the testability of the attachment class:
- Interpolator property is now protected and has accessors/mutators.
- Created getInstanceClass method for returning the class name of the
  attachment's corresponding model instance.

aozisik/stapler,neerajgoyal12/stapler,flaxandteal/stapler,simudream/stapler,CodeSleeve/stapler,riyan8250/stapler,AndreasHeiberg/stapler,InMoji/stapler,oschettler/stapler,sonuku/stapler