When double clicking items on Project panel, they are opened for editing. .uip files are opened in Qt 3DS and so on. Basically the functionality is the same than when double clicking files in the file system, operating system defines which application opens the file.
Exception: for materials, open material for editing in Qt3DS.
It would be nice to add Open with... functionality to context menu so user can choose which program to use for the file format (and hopefully remember the selection in future).