Assume your model is in a file called "foo.3dm"

1) Click on save-> export. When MOI is first started, the dropdown list has 3dm listed as export option.
2) Choose "OBJ" format, and click okay. Your filename will be something like "foo.obj"
3) Cancel out of meshing dialog
4) Go back to save->export.
5) "OBJ" is now selected as default on dropdown in the file dialog, filename is not updated this time to "foo.obj", merely displays "foo"
6) Click save.
7) Moi asks if you want to overwrite foo.3dm
8) If you click yes, moi overwrites the 3DM file with a new 3DM file. If only a part of your current model was selected for export, then the new 3dm file contains only that piece!

So while the dropdown remembers the new default file format to export, the save-dialog/action is not using it when initialized.

This is a nasty lil gotcha!

Another small bug:
File dialog is named "Save As" instead of "Export"