code: function (resolver, fileSrc, callback) {
          // bail out if resolver not found.
          if (!resolver) {
              return callback(new Error('file resolver not supported'));
          }
  
          // bail out if resolver is not supported.
          if (typeof resolver.stat !== FUNCTION || typeof resolver.createReadStream !== FUNCTION) {
              return callback(new Error('file resolver interface mismatch'));
          }
  
          // bail out if file source is invalid or empty string.
          if (!fileSrc || typeof fileSrc !== STRING) {
              return callback(new Error('invalid or missing file source'));
          }
  
          // now that things are sanitized and validated, we transfer it to the
          // stream reading utility function that does the heavy lifting of
          // calling there resolver to return the stream
          return createReadStream(resolver, fileSrc, callback);
      }
docs: Create readable stream for given file as well as detect possible file
  read issues. The resolver also attaches a clone function to the stream
  so that the stream can be restarted any time.
  
  @param {Object} resolver - External file resolver module
  @param {Function} resolver.stat - Resolver method to check for existence and permissions of file
  @param {Function} resolver.createReadStream - Resolver method for creating read stream
  @param {String} fileSrc - File path
  @param {Function} callback -