Attach every image from a multi-file Finder paste #105

Follow-up to #91 and PR #104. That PR attaches pasted images, but a Finder copy of several files attaches only one image.

Confirmed behavior

A standalone WKWebView probe was given a real Finder copy of a.png, b.png and c.txt, all selected in Finder and copied with Cmd+C. The pasteboard held one public.file-url item per file, but the page's paste event saw only:

types: ["Files"], files: ["a.png image/png 14451"], items: ["file image/png"]

• WebKit exposes only the first copied file to clipboardData.files/items and hides the file-name text.
• The Ace-dev build from PR feat(ui): attach images pasted into the composer #104 attached one image for a single file-URL paste, as expected.
• feat(ui): attach images pasted into the composer #104 passes every exposed image file to useAttachments().add in one call, so in-memory image clipboards with several images aren't affected. Only multi-file Finder copies lose files.

Expected

Pasting several images copied in Finder attaches all of them, as dropping them does. This probably needs the desktop shell to read the pasteboard's file URLs natively and hand the image files to the composer. The web clipboard event can't see them.

Related: #26 (non-image uploads). Part of #5.