Solución

let check = setInterval(function() {
  if (new Date() >= channel.trackertimer + 900000) {
    const filename = `${channel.tracker}-${channel.id}-${channel.trackertimer}.txt`;
    const userfriendly = `${msg.author.username}-#${client.channels.get(channel.id).name}.txt`;
    fs.access(filename, fs.constants.R_OK | fs.constants.W_OK, (err) => {
      if (err) return;
      clearInterval(check);
      msg.author.sendFile(filename, userfriendly, `The time ended! There's your file for ${channel}.`)
        .then(() => {
          channel.tracking = false;
          channel.tracker = null;
          channel.trackertimer = null;
          fs.unlink(filename, (err) => {
            if (err) throw err;
          });
        });
    });
  }
}, 1000);