title: I am receiving an error when saving PDF in Expo on iOS
body: <p>I am developing using react-native and the expo client. I have code for saving html that is converted to a PDF. I then save it to the user's phone using the FileSystem.writeAsStringAsync() method. This works on Android but produces the following error on iOS: 'Unhandled promise rejection. This file type is not yet supported'.</p>
  
  <pre><code>let options = {
      html:html,
      base64:true,
    }
    const newPDF = await Print.printToFileAsync(options);
    const base64 = newPDF.uri;
    var path = FileSystem.documentDirectory + 'App.pdf';
  
    if(this.state.downloadType=='qr-codes'){
  
      path = FileSystem.documentDirectory + 'QRCodes.pdf';
  
    }else if(this.state.downloadType=='door-details'){
  
      path = FileSystem.documentDirectory + 'DoorDetails.pdf';
  
    }
  
    await FileSystem.writeAsStringAsync(path, base64);
    const asset = await MediaLibrary.createAssetAsync(path);
    await MediaLibrary.createAlbumAsync("Download", asset, false);
  
    alert("PDF Downloaded Successfully!");
  </code></pre>