Thank you for using our services.
Unfortunately this is not possible with the current dialog. Most of the methods used are either private or internal so overriding the classes used will not help. The easiest approach to this is to go to ~/Sitefnity/Admin/ControlTemplates/Libraries/Dialogs/ItemSelector.ascx and substitute the control which populates the folders with a custom one:
The highlighted control has to be replaced a custom WebUITypeEditor which will return the path to the selected file. You can find example on how to create WebUITypeEditor here
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