Create a new instance of an operation handler for
each project member seems to be an overkill. But
when you share one handler instance per each mime
type you can run into problems.

Usecase:
Let's have a MoveOperation implementation. You
need to perform some postprocessing based on
original filename or location. But in the
MO.afterMove method you have no chance to get the
original file object or any precomputed data
accessible at time before the movement (MO.beforeMove)

Proposal:
The OperationParams class seems to be a good
candidate to  keep status data for handlers during
operations.