How can I get a dependency extension from an DefaultExternalModuleDependency in gradle?

Question

I'm trying to get all the dependencies I've configured in my build in order to solve the problem explained here. My problem is that I have some dependency that have more than one artifact (for example a .zip and a .jar). When I use configurations.compile.allDependencies.each I obtain an object of type DefaultExternalModuleDependency. From this object I can easily get groupId, version and artifact id but it seems not possible to get the type (the extension) that I've defined in the dependencies closure with @zip

How can I get it?

UPDATE:

This is what I'm doing:

dependencies {
    publish 'myorg:myart:1.9.4'
}
...
pom.withXml {
                def dependenciesNode = asNode().appendNode('dependencies')
                configurations.publish.allDependencies.each{
                    ["zip","txt"].each { type ->
                        def dependencyNode = dependenciesNode.appendNode('dependency')
                        dependencyNode.appendNode('groupId', it.group)
                        dependencyNode.appendNode('artifactId', it.name)
                        dependencyNode.appendNode('version', it.version)
                        dependencyNode.appendNode('type', type)
                    }
                }
            }
        }

By now, having always a .zip and a .txt file stored as artifact I resolved with an ["zip","txt"].each closure, but it remains a simplified way.

Thanks, Michele.