Is it possible to write an extension for a specific swift Dictionary ... [String : AnyObject]?

Question

I am trying to write an extension for this:

extension [String : AnyObject] {

}

It throws and error. I was also trying to come up with something similar as this:

extension Dictionary: [String : AnyObject] {

}

It also fails.

Is there a solution that works similar as this (I can't remember proper syntax)?

extension Dictionary where ?? is [String : AnyObject] {

}