What files can I upload, and how do I get the Garmin one?

Three kinds of file: a Garmin "Export Your Data" .zip, your Strava archive .zip, and .fit, .gpx or .tcx activity files from any watch or app, one session per file: on their own (gzipped as .gz too) or up to 50 together in one .zip. So a Suunto, a COROS or anything else that can export a session has a way in. Other files, such as .csv or a Fitbit export, cannot be read yet, and the uploader tells you so instead of importing nothing.

To get the Garmin file, request it from Garmin's account portal ("Export Your Data"). Garmin emails a download link; upload the resulting .zip as-is, with no need to unpack it.