Reset property so it doesn't leak to other models

If the FileValidation is used in multiple models which are validated
within a single request, since Behaviours are effectively singletons,
their internal state leaks over to the next model.

This prevents $_tmpFile still being set for the next model; otherwise
this file would be tried to be deleted again in the next model validation
which does not exist anymore.
Merge pull request #186 from mfn/mf-filevalidation-reuse

Reset property so it doesn't leak to other models