file-storage.js

var STORAGE = {};

function createFunctionPromise(func, args) {
  return new Promise(function(resolve, reject) {
    var data = func.apply(null, args);

    if (data instanceof Promise) {
      return data.then(resolve, reject);
    }

    return resolve(data);
  });
}

function createLoadFilePromise(fs, filepath) {
  return new Promise(function(resolve, reject) {
    fs.readFile(filepath, function(err, data) {
      if (err) {
        return reject(err);
      }

      return resolve(data);
    });
  });
}

module.exports = function(fs) {
  function register(key, payload) {
    if (payload instanceof Promise) {
      STORAGE[key] = payload;
    }
    if (payload instanceof ArrayBuffer || (global.Buffer && payload instanceof global.Buffer)) {
      STORAGE[key] = Promise.resolve(payload);
    }
    if (typeof payload === "function") {
      STORAGE[key] = createFunctionPromise(payload, [].slice.call(arguments, 2));
    }
    if (typeof payload === "string") {
      STORAGE[key] = createLoadFilePromise(fs, payload);
    }
  }

  function unregister(key) {
    if (STORAGE.hasOwnProperty(key)) {
      delete STORAGE[key];
    }
  }

  function load(key) {
    if (STORAGE.hasOwnProperty(key)) {
      return STORAGE[key];
    }
    return Promise.reject(new TypeError("NOT FOUND: " + key));
  }

  return { register: register, unregister: unregister, load: load };
};

usage.js

const fs = require("fs");
const FileStorage = require("file-storage")(fs);

FileStorage.register("a", new Buffer("abc"));
FileStorage.register("b", "./b.txt");
FileStorage.register("c", loadFromMemcached, "c");

//
FileStorage.load("a").then((data) => {
  assert.deepEqual(data, new Buffer("abc"));
});