title: Preview multiple drop using react-dropzone
body: <p>I'm trying to implement Dropzone on my app but I can't preview photos if they are drop as a multiple input. If I add them one by one it works fine but if I select multiple only the first one gets rendered. </p>
  
  <p>This is my onDrop function</p>
  
  <pre><code>onDropGeneral = (currentGeneralPhoto) =&gt; {
   let index;
   for (index = 0; index &lt; currentGeneralPhoto.length; ++index) {
    const file = currentGeneralPhoto[index];
    this.setState({
      previewGeneralPhotos: this.state.previewGeneralPhotos.concat(file)
    });
    const reader = new FileReader();
    reader.readAsDataURL(file);
    reader.onload = (event) =&gt; {
      console.log('URL: ', event.target.result);
      this.setState({
        generalPhotos: this.state.generalPhotos.concat([{ base64: event.target.result }])
      });
    };
   }
  }
  </code></pre>
  
  <p>and this is my render method:</p>
  
  <pre><code>&lt;h2&gt;Dropped files&lt;/h2&gt;
  {this.state.previewGeneralPhotos.length &gt; 0 ? &lt;div&gt;
    &lt;h2&gt;Preview {this.state.previewGeneralPhotos.length} files...&lt;/h2&gt;
    &lt;div&gt;{this.state.previewGeneralPhotos.map((file) =&gt; &lt;img src={file.preview} alt="preview failed" /&gt;)}&lt;/div&gt;
   &lt;/div&gt; : null}
   &lt;h2&gt; Upload {this.state.generalPhotos.length} Files &lt;/h2&gt;
  </code></pre>
  
  <p>the Upload count is showing the right size of the array but the Preview count only counts the 1st photo dropped </p>