[External Document Picker] Introduce "icon.style" parameter in the external document picker plugin

Currently the dimensions of the icon displayed in the external picker dialog cannot be configured: in case of images with size bigger than the container, they will exceed the available space. 

It would be a nice addition the introduction of the "icon.style" parameter: this should resize the icon to fit inside the container with the setting: WIDTH: 20px; HEIGHT: 20px for example.