Are you by any chance using the FileExplorer control in a FileTree
ExplorerMode? If so, I am afraid that this behavior is expected as the "Open" option is part only of the Grid/ListView control's context menu. As a workaround for this behavior I can suggest that you implement a custom TreeView's context menu button like explained in this help article: Adding a custom button to the toolbar and an item to the tree context menu
If this is not the case, though, can you please provide some more details on the specific scenario in which you are using the control? Can you send us the problematic FileExplrorer's configurartion so we can examine it on our side?