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If you click on the filename shown at the top of the window it will open an interface to rename the file. If the file extension doesn't show there, then you have the extension set to be hidden in Finder. Other options would be to command-click on the same place (the filename) and this will open a menu showing you the folder hierarchy the file is stored in. You can click on the top menu to open that folder in Finder to quickly reach the file. You can also use File > Export and export to the OmniOutliner 3 format, however, you seem to have multiple extensions so fixing this in Finder or through the rename interface is probably the best option.