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To share a specific folder, select the "Share" option in the Dropdown menu of the "Actions" button in the folder.
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In Tonido iOS v4.0 onwards, the iPad version will allow jumping to base folder by tapping the "Files" option on the left column view.
In Tonido iOS v4.0.1 onwards, the iPhone version will allow jumping to the base folder by tapping the "Files" option on the bottom bar.