The Explode validator requires a child validator for it to work. The child validator is impossible to add via the InputFilter Factory (and Annotations). This affects Select, MultiCheckbox, Email, and File elements.
A potential solution might be to change the Explode validator to extend from ValidatorChain, instead of AbstractValidator.
@cgmartin is this still an issue?
@weierophinney this seems to be a form / annotation issue as well? can you examine?
Well, it's related to the factory. I like the idea @cgmartin presents -- what I'd like to see is the desired code and/or configuration to get the behavior; from there, I can create a solution.
I'd be happy to come up with some code, configuration, and take a stab at a solution. Will add this one to my todo list.
Closing due to inactivity.