strava_upload_solution.js

const body = new FormData();

body.append('name', activity_name);
body.append('data_type', 'gpx');
body.append('external_id', activity_id);
body.append(
  'file',
  Buffer.from(gpxFileString, 'utf-8'),
  `activity_${activity_id}.gpx`
);

const stravaUploadResponse = await fetch(
  'https://www.strava.com/api/v3/uploads',
  {
    method: 'POST',
    headers: {
      Authorization: `Bearer ${newTokens.access_token}`,
      ...body.getHeaders(),
    },
    body,
  }
).then(response => response.json());