No client (Mail included) currently exercises these codepaths. In subsequent patches, I'm reimplementing this feature in a way that is (1) compatible with dragging on iOS, and (2) is generalizable to non-file-backed blobs (i.e. Files that are created entirely via bindings, and thus have no references to file paths on the system).
Created attachment 330258 [details] Patch
Created attachment 330259 [details] Fix Win/GTK builds
Comment on attachment 330259 [details] Fix Win/GTK builds Thanks Dan!
Comment on attachment 330259 [details] Fix Win/GTK builds Clearing flags on attachment: 330259 Committed r226312: <https://trac.webkit.org/changeset/226312>
All reviewed patches have been landed. Closing bug.
<rdar://problem/36260924>