docs: function createMultipartUpload (req, res, next) {
      // @ts-ignore The `uppy` property is added by middleware before reaching here.
      const client = req.uppy.s3Client
      const key = config.getKey(req, req.body.filename)
      const { type } = req.body
      if (typeof key !== 'string') {
        return res.status(500).json({ error: 's3: filename returned from `getKey` must be a string' })
      }
      if (typeof type !== 'string') {
        return res.status(400).json({ error: 's3: content type must be a string' })
      }
  
      client.createMultipartUpload({
        Bucket: config.bucket,
        Key: key,
        ACL: config.acl,
        ContentType: type,
        Expires: ms('5 minutes') / 1000
      }, (err, data) => {
        if (err) {
          next(err)
          return
        }
        res.json({
          key: data.Key,
          uploadId: data.UploadId
        })
      })
    }