storefile.js

var path = require("path");

exports.media = function(req, res, next) {

  var file = req.files.media_upload;
  // code to handle file here:
  // file should already be in the right location
  // The path, this file has been saved at should be:
  file.path;
  // To get only the filename of this path:
  var filename = path.basename(file.path);

  Project.findById(req.params.id, function(err, project) {
    // First make sure that media is actually an array, not undefined.
    if (!project.media) { project.media = [ ]; }
    // Now add the filename
    project.media.push(filename);
    // And save it
    project.save(function(err) {
      if (err) { return next(err); }
      res.send("success");
    });
  });

};