Looks like that is how Electron's openFileDialog behaves (just reproduced it with Electron Fiddle on Windows). When there is a list of supported types (All, CSV, JSON in our case) for some reason on Windows it defaults to JSON and there doesn't seem to be a way to affect that or to explicitly se a default.

Not tying the file selection dialog to the type that was selected in the UI was a design choice. In fact, even if you don't select the type in the UI but just select a file (file selection comes first in the import dialog) the type will be detected automatically.