Dragging a folder from Explorer/Finder into the composer failed with "file
not found on gateway and no data_url provided", on local gateways too.
extractDroppedFiles tagged every OS drop as a File-bearing entry, so
partitionDroppedFiles routed the folder to the upload pipeline and
file.attach tried to read a directory's bytes — a directory has none, and
there is no data_url to send. This regressed in 4906dcfc25, which routed
OS drops through file.attach to reach a remote gateway but did not exclude
directories, which also carry a File handle.
Detect directories at drop time with DataTransferItem.webkitGetAsEntry(),
the only synchronous way to tell a dropped folder from a file. A dropped
directory now becomes a path-only entry with isDirectory set, which routes
to a @folder: ref exactly like the folder picker, instead of the file
upload path that cannot stage a directory.
Process transfer.items before transfer.files: webkitGetAsEntry lives only
on items, and claiming the folder's path there first lets the files
fallback dedup skip the same entry (Chromium lists a dropped folder in
both). Path-based dedup and the getPathForFile resolution are preserved.