When dropping a file or folder from Finder into an attachment-element-enabled web view, WebKit immediately initializes an NSFileWrapper instance with the dropped item’s URL. This can fail in some cases, including when the file comes from iCloud Drive and is not yet available locally. WebKit should use NSFileCoordinator to coordinate reading from the URL, and initialize the NSFileWrapper with the URL passed to the accessor block. It may also need to expose _WKAttachment and/or WKUIDelegatePrivate API to inform clients when an attachment’s file wrapper becomes available (or changes), and have a way to represent an attachment pending coordinated reading.
<rdar://problem/45672625>
This can probably use the “file wrapper invalidated” mechanism that was added for editable images.