server control property editor

[laptop]

hello:
using vb.net - I have a server control.
I have a property but I want to have a file open editor attached to it.

<Bindable(True), Category("Custom Properties"), Editor("?")
Localizable(True), Description("Image Source"), DisplayName("Image Source")>
Property ImageSource() As String



What should I put in the editor so that in design mode, the user can click
on and have a file-opn experience.

Thanks