You can now upload .fit files for completed activities. Laps are not imported yet but other stuff should work. I have only tested this on files from my Garmin Edge 130+ and my wife’s Garmin Forerunner 230.
I am busy with other related stuff like making it possible to delete activities from Strava on Intervals.icu only, add manual activities on Intervals.icu without having to do so on Strava etc…
You can download the original uploaded file on the activity data page: