constructor(fileHandlerCallback) {
    const dropZone = document.querySelector('.drop_zone');
    if (!dropZone) {
      throw new Error('Drag and drop `.drop_zone` element not found in page');
    }

    this._dropZone = dropZone;
    this._fileHandlerCallback = fileHandlerCallback;
    this._dragging = false;

    this._addListeners();
  }
@param {function(string): void} fileHandlerCallback Invoked when the user chooses a new file.