Allowing only 'copy' for file attachment drags kept File Explorer from
moving files out of 'storage' but locked the cursor at '+' even for
moves within Zotero. Instead, provide the file via a flavor data
provider that copies it to the temp directory when a target asks for it,
so the drag can allow 'copyMove' again and Explorer's move only touches
the copy. A copy's directory is removed once Explorer has moved the file
out of it, at the next drag, or with the temp directory at shutdown.
