Can't load preview images for shared PDFs

[ole]

Steps to reproduce:

1. Open Safari in iOS 13 and open a web page.
2. Open the share sheet and tap Options > Send as PDF
3. Share to Share Inspector. The preview image cell displays an error.

(See #1 for a screenshot.)

[ole]

This seems to be an issue with Safari and not a ShareInspector bug. NSItemProvider.loadPreviewImage returns an error when loading the preview image for a PDF in Safari:

Error Domain=NSItemProviderErrorDomain Code=-1000 "Cannot load preview." UserInfo={NSLocalizedDescription=Cannot load preview.}

We can't do anything about this.

On the other hand, sharing a PDF file from the Files app doesn't provide a preview image at all (but also no error), and sharing a PDF file from PSPDFKit's PDF Viewer app does provide a working preview image.