code: function verifySize(correctSize, filepath) {
      return calculateSize(filepath).then(size => {
          if (size !== correctSize) {
              throw new Error(`Incorrect file size: expected ${correctSize}, got ${size}`);
          }
          return true;
      });
  }
docs: Calculates file size and compares
  it to the provided one.
  
  Returns a promise which rejects if the size is
  incorrect, otherwise resolves to true.
  
  @param {number} correctSize expected file size
  @param {string} filepath file path
  @returns {Promise<boolean>}