data-transfer-item-get-as-string-wrapper.js

  function getMyString(datum)
  {
    let promise = new Promise(function(resolve, reject) 
    {
      // if readonly mode, reject();
      // per https://www.w3.org/TR/html51/editing.html#the-datatransferitem-interface

      if datum.kind !== 'string'
        reject();
      else
        // We can't seem to test for 'readonly mode' and don't know about other
        // failures, so we still need a timeout to reject.
        
        setTimeout(function(){console.log('timeout reject from getMyString'); reject()}, 1000);
        datum.getAsString(resolve);
    });  
  }
}

function getImageURL(data) {
  const urlList = data.find(d => (data.kind == 'string') && (data.type.match('^text/uri-list'))
    
  if (urlList) 
  {
    return getMyString(urlList);
  } else {
    return Promise.resolve();
  }
}